Five Nights at Miku's 3 is expected to be at least 2-3x bigger than Five Nights at Miku's 2, as the game is divided into 4 missions.

Mission A - The Forest: This is the first part of the game and if you know what you're doing, it should take no longer than perhaps 10-15 minutes.

Mission B - The Cave: This mission is divided into 2 parts, the first of which being 6 minutes long, while the second part will be 3 minutes long.

Mission C - Five Nights at Miku's: This mission is obviously the main game and it's 5 nights, with each night being 300 seconds long. Without deaths or counting any post-night sequences, Mission C is 25 minutes long.

Mission D - ????????: As this is the endgame, I cannot go into any details. Plus, at this point in time, it's not in development yet.

And that's not even mentioning the two branching paths for Mission C and all the extra content!