Estimate time to 100% a game.
Full completion is a multiple of the main story, and that multiple varies enormously by genre: linear action games sit near 1.5x, while open-world titles with collectibles routinely reach three or four times the story length. Applying the multiplier gives the true target, scaling by current progress gives what is left, and dividing by weekly play hours turns it into a realistic calendar rather than an intention.
Full completion target
Total hours = main story hours x completionist multiplier
Remaining work
Hours remaining = total hours x (1 - current completion); weeks = hours remaining / weekly play hours
Community tracking sites publish story and completionist averages side by side for most titles. Dividing one by the other gives the multiplier directly.
Because it is usually the grind: rare drops, difficulty-gated challenges and collectibles with no in-game markers. Progress percentage is not linear in time.