Isn't the game's quality more important than its length? Chrono Trigger might well be the best JRPG ever made and it's only around 25-30 hours. Frankly, for as much as I enjoyed Persona 3 FES, I would be far more likely to replay it if its length fell below the fifty hour mark. Much of that length was possible because both Persona games include small areas that you run through many, many times, random dungeon floors consisting of pre-made tiles, and boatloads of writing and dialogue.
I can't say that I know what to expect from Persona 5. I'm pretty sure it's going to include everything that makes a Persona game (social links, dungeon-crawling at night, etc.). Or, at least, what made Persona 3 and 4 - I don't think the first and second games had those things. For right now, I mostly want to know what this game looks like. Is it a more realistic-looking anime style? Are the dungeons random or pre-designed? Are we going to get anime cutscenes or pre-rendered ones?