FFVII isn't a speed game! It's a 'brag to your friend that you have over 100 hours of gameplay on your clock because you have no life' kinda game.
I usually have every character to level 99, all limit breaks learnt (with the exception of one... *wink*), all materias, all master materias (without cheating with the guy in kalm), all ultimate weapons... Oh yeah, and a Chocobo stables full of goldies!
The sad thing is, by the time I've done all that, I forget I even have friends, so I have to brag about it to Vincent...
Although getting back on topic, there's supposed to be like what, 20 hours of cut scenes and general loading times, so I'd say realistically speaking, it would be possible in about 35 hours ish. But then again, I've never played for speed, so I couldn't really say.
Ps: I know my estimate is high, but I'm still thinkin of getting some sub missions done, maybe going for a final limit break or two, before defeating the big man in the crater.