Honestly, depends on the game. Longer is not always better, and I think the artificial lengthening of games has led to some decreased quality in AAA titles. Persona 5, in my opinion, is far too long and drawn out. I think P5 should be about two dungeons shorter. On the other hand, Persona 4 is about as long but never felt as if it was dragging along. Xenoblade Chronicles is another epic, long game that never overstays its welcome.
Short games can be just as captivating. Naughty Dog's third-person shooters are some of the most acclaimed games of all time, and all of them can be finished within 15 hours.