It certainly depends to the enjoyment one can have from the game!
In a single playthrough, I can remember going for more than 100 hours in FFX, FFXIII and Resonance of Fate. I may be forgetting a few SRPGs but even doing everything I wouldn't say older games were shorter but they certainly had some easier ways to get money, XP or items that would destroy the game fast.
In FFVII the Knights of the Round materia for example made everything else obsolete. After getting it, with the exception of the Ruby and Emerald weapons everything else is a cakewalk.
My point is that for me, it normally comes down to how fast I can do the optional stuf. If I want to go further, I have to set personal goes like, maxing all skills, getting all magic, items, leveling everyone to max level or something of the sort.
Honestly, I have a friend, who isn't much a fan of RPGs, and it starting to become one after playing (believe it or not) FFXIII. The thing is he was always scared of how much time the game could take. I say that as long as it's fun it is worth playing.