How long does Scott languish in the Quantum Realm between Ant-Man when Michael Douglas' Hank Pym warned his successor never to go subatomic. Paul Rudd's character does so anyway, and finds a way back into the real world. This neatly sets up the events of Ant Man & The Wasp, where Scott, Hank and Hope van Dyne work together to bring the original Wasp, presumed lost following a 1987 mission gone wrong, back from the Quantum Realm.
Now that Team Ant-Man are able to enter and exit the Quantum Realm in relative safety, they continue their subatomic experiments into the sequel's mid-credits scene. Unfortunately, Thanos executes his snap while Scott is venturing in the Quantum Realm, and his three allies all turn to dust, leaving Ant-Man with no way out since the van's control mechanism can only be operated from the outside. Unable to return to normal size, Scott is listed as another snap victim, as confirmed by Avengers: Endgame. And that's the way things would've stayed were it not for the rat who literally saved the universe by tripping the Quantum Realm exit switch while Scott's van was impounded. Finally, Ant-Man tumbles out of the subatomic gate and returns to the real world, but how long was he missing?
The mid-credits scene of Ant-Man & The Wasp must coincide with the Battle of Wakanda in Quantum Realm.
And "hours" is precisely the point, as Scott's absence was much shorter for him than it was for his loved ones. After escaping from the van and seeking out his daughter, Scott does the sensible thing and calls on the Avengers... or what's left of them. Discussing a potential way of undoing the damage Thanos caused, Scott floats the idea of using the Quantum Realm to go back in time, revealing to a stunned Black Widow and Captain America, "for me, it was five hours." Given Lang's notoriously short attention span, he was probably at his wits' end spending that long in the Quantum Realm alone, but he should actually consider himself lucky.
Time is not a fixed construct in the Quantum Realm; nothing flows with the same regularity as the real world, so there's no definitive rule that one hour spent in the micro-world equates to one year in the real world. We know this because when Janet van Dyne inhabits Scott's in Ant-Man & The Wasp, she says "trust me, after 30 years down here." This line implies that just as 30 years or so ed for Hank and Hope, from 1987 to 2018, the same amount of time went by for Janet in the Quantum Realm. Ant-Man was simply fortunate that the 5 years he was trapped felt like a relatively short sentence from his perspective.
Ant-Man's five hours in the Quantum Realm is a useful story mechanic for Avengers: Endgame. Due to the film's time skip, the audience are left feeling somewhat unfamiliar with the likes of Captain America, Thor, Hulk and the other survivors, who have all changed greatly during that time. Experiencing the new world through the eyes of Scott Lang (whose mindset is still in the time of Avengers: Infinity War) gives viewers a com point to follow through the new world, where Ant-Man is just as confused and shocked as they are.
