That's why I think it will end in a draw, no other reason to have a time limit as a rule imo... heck I guess it might be a close call, like someone winning in the last second or something, but I think it's far more likely to have a draw between jiren, hit and U7 with Goku/Gohan/Vegeta(I can totally see Gohan being the last one, seeing as he's the only one with a brain there).
Now what happens if there's a draw, I have no idea, maybe they all get to have their universes survive and then forced to continue until only one is left, to see who gets the Super Dragon Balls wish, but without getting their universes erased.