FFXII took the longest, barely edging out Tactics. Even when doing a fair amount of sidequests first, none of the other FF's have taken much more than 40 hours tops to beat. If I did all of the sidequests that would be different. The older ones are much shorter though. FFVI wouldn't take me much longer than 30 hours doing everything. I could finish FFIV in under 20 without much trouble.