As others said, 20-30 hours is best, around the same time an average book takes to be read.

I believe it is not possible (or very hard) to go beyond 50 hours without putting in cyclic gameplay, which is a bit cheating. Towers and all are fine, but games tend to abuse theses mecanics to make a game unfinishable in less than 100-150 hours.

Under 10 hours to complete the game, like an experience is perfect for me. Not everyone has a lot of time to play games. If the game is good, i have a nice time and can replay it, if it's meh, i can satisfy my curiosity by finishing it and move on something else. Indie games best capture theses.

Again, there are exceptions. I've been playing The Legend of Zelda Breath of The Wild for more than 200 hours with my family and I still enjoy it. Same for Minecraft, i guess.