The relationships in Harry Potter ranged from short flings to strong bonds that last decades. A few of these relationships revolved around the formal, fancy Yule Ball, where the characters asked each other out and spent time with one another during a special event. However, almost all of them began during the characters' time at Hogwarts.
Some relationships are carried out during undefined periods of time, such as Draco Malfoy's marriage to Astoria Greengrass and Neville Longbottom's marriage to Hannah Abbott, both of which take place after the series and are not given a specific timeline. However, many of these relationships are observed during the present tense narrative of the franchise, allowing audiences to track how long they lasted.
Harry Potter & Cho Chang - 4 months
Harry developed feelings for Cho Chang in his fourth year at Hogwarts and even asked her to the Yule Ball during the Triwizard Tournament. However, Cho was dating Cedric Diggory at the time. Harry and Cho spend more time together during their fifth year, attending Dumbledore's Army meetings together, and they shared their first kiss in the Room of Requirement beneath the mistletoe.
The relationship lasted from December to March of the next year. Sadly, the couple ended up breaking up when Harry believed that Cho betrayed Dumbledore's Army to Umbridge.
Ron Weasley & Lavender Brown - 5 months
After Ron discovers that Hermione does not believe in his quidditch abilities, he storms off and begins a shallow relationship with Lavender Brown. Lavender is fanatic about Ron, and they often "snog" around Hogwarts, disgusting their friends.
Ron and Lavender start dating before Christmas time, and Lavender sends Ron a necklace for Christmas, which he hates. The necklace opens Ron's eyes to the shallow nature of the relationship, as they don't know anything about each other, and Ron is really only using her as a distraction. Ron quickly grows bored of her and takes to ignoring her completely. They break up in April when Harry takes the liquid luck Felix Felicis potion and accidentally causes a misunderstanding between the two of them.
Hermione Granger & Viktor Krum - Around 7 months
Viktor asked Hermione to be his date to the Yule Ball in December of Hermione's fourth year at Hogwarts, making Ron extremely jealous. In the Goblet of Fire book, Ginny gossips that Viktor and Hermione kissed after the dance, angering Ron even further, and Hermione is upset that Ron will not support her because of his jealousy.
Viktor and Hermione date until summer, when Viktor leaves to go home. Hogwarts' terms end in June, which may have been the end date for their relationship. Viktor asks Hermione to visit him over the summer, but it is unlikely that she did, and we are to assume that the romance ended.
Cho Chang & Cedric Diggory - 7 months
Cedric asked Cho to the Yule Ball in their fourth year at Hogwarts, and they began dating around December. Cho was taken as his hostage in the Triwizard Tournament since he had strong feelings for her, which she reciprocated. However, Cedric was murdered by Voldemort in June, 7 months after he and Cho started dating.
During their relationship, Cedric and Cho seem to have deep feelings for each other, especially given how heartbroken Cho is after Cedric's death. Despite Cedric being framed as a rival to Harry, he was presumably a good boyfriend to Cho.
Ginny Weasley & Dean Thomas - 11 months
Ginny starts dating Dean at the end of her fourth year, around June 1996. They have a rocky relationship since Dean constantly tries to help Ginny in small ways and Ginny wants to be more independent as she grows older. Ginny also isn't very excited to see Dean when they return to Hogwarts after the 1996 Christmas break, indicating that their relationship may be fizzling out.
Harry accidentally incites the argument that leads to their breakup in April 1997, when (in his invisibility cloak) he brushes against Ginny's shoulder and she thinks that Dean is once again trying to help her. Ginny also has feelings for Harry and ends up dating Harry a few weeks after her breakup with Dean. Dean does not take this news well, smashing a glass when he sees them together.
Nymphadora Tonks & Remus Lupin - around 1 year
In the books, Tonks declares her love for Lupin a few weeks after Dumbledore's death. Although Lupin feels close to her, he is initially wary and cowardly, refusing her advances, as he doesn't believe anyone could love someone like him. However, they eventually marry in a quiet summer ceremony in 1997 and Tonks becomes pregnant with their son Teddy. During their relationship, Tonks loved Lupin so much that her Patronus changed to a wolf.
Both Tonks and Lupin are killed in the battle of Hogwarts, May 1998, leaving Teddy in the care of his grandmother Andromeda and his godfather Harry Potter - and effectively ending their relationship, for obvious reasons.
Lily Evans & James Potter - 4 years
This pairing got off to a rocky start, as Lily was disgusted with James' delight in hexing other students for fun. James' bullying of Severus Snape was particularly repulsive to Lily. However, James and Lily developed feelings for each other over their time at Hogwarts. They became Head Girl and Head Boy respectively in 1977 and started dating the year after.
They married sometime between summer 1978 and autumn 1979. Sadly, their relationship tragically ended in 1981 when they sacrificed themselves to save their son, Harry, from Lord Voldemort.
Ron Weasley & Hermione Granger - 19 years
Ron and Hermione were best friends from their first year at Hogwarts together, and despite their constant bickering, they developed feelings for each other and even tried to make the other jealous.
Their relationship in the books began in 1997 when they "sort of" date during Bill and Fleur's wedding, however, in the films, their first kiss is during the battle of Hogwarts, 1998. Ron and Hermione eventually marry and have two children, Hugo and Rose. Their relationship reaches 19 years as of 2017, the end of the final book.
Harry Potter & Ginny Weasley - 20 years
Ginny harbored a crush on Harry ever since their first meeting at Platform 9 and 3/4. In the books, Ginny and Harry enjoy Quidditch together, fighting alongside each other when needed. It was in Harry's fifth year that he started to develop feelings for Ginny, seeing her as a confident young woman rather than just his best friend's little sister.
Ginny and Harry start dating in 1997, during the events of The Half-Blood Prince. In the films, they keep dating, but in the books, they briefly break up before the Battle of Hogwarts because Harry is afraid Voldemort will harm Ginny to get to him, as Tom Riddle did in The Chamber of Secrets. Their relationship reaches 20 years as of 2017, the end of the final book.
Bill Weasley & Fleur Delacour - 22 years
Fleur was interested in Bill during the Triwizard Tournament, watching him from across the room. In the summer of 1995, Fleur and Bill both worked at Gringotts and immediately began dating. The Weasleys were not happy with the match, as Fleur seemed stuck-up and posh. However, when Bill was injured by the werewolf Fenrir Greyback, Fleur proved to their families that her love was not skin-deep, staying by Bill's side to help his recovery.
The pair got married in 1997 and fought together in the Battle of Hogwarts alongside the other Weasleys. They had three children, Victoire, Dominique, and Louis. Their relationship reaches 22 years as of 2017, the end of the final book.
Arthur Weasley & Molly Weasley - around 56 years
Molly Prewett and Arthur Weasley met at some time in their Hogwarts years, from 1961 to 1968. They often got in trouble together, for example walking around the lake together after curfew. They married shortly after graduation, presumably in 1968 as their marriage was rushed to take place before Voldemort's Wizarding War.
Their first child Bill was born in 1970. Arthur and Molly have one of the strongest relationships in the series and prove over and over again that their bond with their children is irreplaceable, as they would do anything for their family. The Weasleys are kind, accepting Harry into their family without a second thought, and they are shown to be deeply in love throughout the series.