I suppose it ultimately depends on the pacing the game wants to achieve for itself, but I think a good milestone is probably within the first three hours of gaming, at a maximum. If the game uses a mission format, then having a boss at the end of the first mission (which often doubles as the tutorial) is a good idea.

I like how Dragon Age: Inquisition set it up: the first mission teaches you the basics of game-play as you travel through the valley fighting demons. Then at the end, the boss demon appears which tests you (at varying levels of difficulty depending on what difficulty you've set) on what's you've learnt. I think that's a good set-up.

Kingdom Hearts 'kinda' has the boss right at the start. There's one or two fights with shadows, then you're fighting the Darkside, though not before you're told the basics of combat, so it isn't *right* at the start, but it basically is pretty much at the start. I think it worked well enough; you don't actually have to win that battle.