For what could be his final Transformers film, Michael Bay could have have crafted a lengthy conclusion in Transformers: The Last Knight. Since Paramount handed the reins to Bay and saw him launch the franchise in 2007, the Transformers films have progressively grown longer. The first trilogy all hovered around 2 hours and 30 minutes, but it was the latest effort, Transformers: Age of Extinction, that saw its runtime increase to 2 hours and 45 minutes full of explosions and Transformers.
Based on what we know of The Last Knight, Bay has a complex story to unpack in what he says should be his final (again) directorial effort in the franchise. With Optimus Prime turning evil, Nazis, King Arthur and his Excalibur sword, and possibly Unicron all in the mix, Bay could easily fill out his typical timeframe. Depending on which runtime is to be believed, The Last Knight could come in as the longest one yet. Update: But it's not.
While a simple Google search for "Transformers 5 runtime" will bring up a result indicating that the film is 3 hours and 2 minutes long, there's no clear source on this information. The same goes for Cineworld which listed a 2 hour and 30 minute runtime in the information section when trying to buy tickets. Since this is making news headlines, Michael Bay commented to clarify that Transformers: The Last Knight is actually shorter than the last few:
Reports of #transformers The Last Knight being over 3hrs is wrong. It's shorter than the last 3 movies by a lot.
— Michael Bay (@michaelbay) June 1, 2017
Based on the previous track record for this franchise - and most Hollywood blockbusters - a two and a half hour runtime would be more believable at this point. It would not only follow suit with previous films, but also help solve one of the criticisms of Age of Extinction, that being how long and drawn out the story and action was.
As the release gets closer, it will be made official just how long of a Transformers film Bay and Paramount have planned for the fifth film. While studios are typically wary of releasing films with similarly long three-hour runtimes (with an example being Warner Bros. cutting Batman V Superman: Dawn of Justice down to 2 hours 31 minutes), this franchise has shown the ability to continually draw in mountains of cash - especially overseas - so Paramount would allow Bay to make a three hour epic if that's what he desired. It's not the case on this one though.
Sources: Google, Paramount, Michael BayHarley Quinn & Joker's Romance Confused Margot Robbie, Too