Hm... Lets do some guesswork:

FFI: 30 hours
FFII: 10 hrs
FFIII: 30 hrs
FFIV: 5 x40 = 200 hrs
FFV: 4 x 45 = 280 hrs
FFVI: 6 x 60 = 360 hrs
FFVII: (3 x 120) + (5 x 60) + (1 x 24) = 684 hrs
FFVIII: 60 hrs? How long does it take to get to the end game in FFVIII?
FFIX: (3 x 50) + (1 x 80) = 230 hrs
FFX: 100 + 30 = 130 hrs
FFXII: 80 hrs
FFXIII: 40 hrs
FFT: (2 x 160) + (2 x 120) + (4 x 50) = 760 hrs
FF Subtotal: 2896
Chrono Trigger: (5 x 50) + (12 x 20) = 490 hrs
Chrono Cross: 4 x 40 = 160 hrs
Xenogears: 2 x 40 = 80 hrs? How long is xeno again?
Vagrant Story: 20 hrs
Infinite Undiscovery: 10 hrs
Last Remnant: 20 hrs
Random Stuff: 100 hrs
Non FF Subtotal: 880

Total-ish: 3776
I think I may have overestimated the length on a few of the titles though. I think by doing things this way you can get a very clear picture of which games are really your favorites.

so I was able to add em all up. 3,756 hours, 24 min, 18 second
Cool, less than a percentage difference.