Here's each time Captain America proclaims "I can do this all day" in the MCU, and the context behind each one. The first Captain America movie came in the form of a 1979 TV film starring Reb Brown, which was quickly followed by sequel Captain American II: Death Too Soon. These films were cheaply made and played fast and loose with the source material, and a planned TV series failed to follow. The second attempt was 1990's Captain America from Cannon, which starred J.D. Salinger's son Matt in the title role.
The movie was produced on a modest budget and wound up going straight to VHS, where it received terrible reviews. The character finally got his due with 2011's Captain America: The First Avenger starring Chris Evans, which introduced the character to the MCU, where he became a major figure. Evans returned for two direct sequels, in addition to The Avengers movies and some assorted cameos in other MCU efforts.
While there was concern the character would be too corny or outdated for modern viewers, Evans' winning performance and some smart writing made Captain America one of the highlights of the MCU. His catchphrase became "I can do this day," and here's a rundown of every time he says it.
Captain America: The First Avenger
Steve Rogers starts Captain America: The First Avenger as a frail young man who wants to do his part in the war, but is rejected due to various medical issues. During an early scene he shouts at a heckler in a movie screening to be quiet, who subsequently takes him outside to beat him up. Steve keeps getting back up, however, and when the bully tells him he doesn't know when to quit, Steve proclaims "I can do this all day."
After he's taken the serum to become Captain America and led the fight against the forces of Red Skull and HYDRA, he later finds himself being beaten up by the villain himself. Revealing he's still the same old Steve despite the new superpowers, he defiantly repeats his catchphrase.
Captain America: Civil War
Captain America: Civil War from 2016 ends with Steve and Bucky in a fight against Tony Stark, after the latter learns the brainwashed Bucky killed his parents years before. As the battle is winding down Tony gains the upper hand and brutally beats Steve to get to Bucky, and warns him to stay down. Captain America raises shakily to his feet and declares "I can do this all day."
Avengers: Endgame
A very meta moment in Avengers: Endgame has Steve come face to face with himself from The Avengers 2012 era, who believes modern-day Steve is Loki in disguise. They fight, and the younger version of Captain America gets the upper hand, stating "I can do this all day." The older, more weary Steve just declares "Yeah I know, I know."