Its over when the media says its over.
I think Trump will drag it out beyond that point. Even if the media turns against him and stops granting him airtime, he still has his own communication channels. The man will never, ever accept that he's been defeated in anything, or that he's been wrong in anything. Can you imagine him admitting that he called wrong on even the most insignificant event, such as a coin toss or a dice roll? Or that he lost in a golf match? Let alone that he's come second in a two-horse race for leader-of-the-free-world, with billions of people watching on.
For Biden supporters the election has two possible outcomes:
Biden wins
Biden loses
For Trump supporters* the election has two possible outcomes:
Trump wins
Trump wins but is cheated out of victory
It's absurd. The problem is that a proportion of the Trump base are backing him through faith rather than reason. That's why facts have no effect on them.
Of course some benighted countries do have elections where the result is determined prior to ballots being counted. America really shouldn't want to become one of those countries.
*
Not all Trump supporters, obviously. But the loudest ones tend to grab the attention.---
Further to this, the fallout could get messy and violent. Will he leave the White House quietly? Will he encourage armed extremist nut-jobs to vote with their guns? I think it's a genuine concern. It's difficult to rule anything out at this point. The Biden campaign's tongue-in-cheek comment that "the United States government is perfectly capable of escorting trespassers out of the White House" could yet come to pass.