How many times?

IV: Twice
V: One and a half
VI: Twice or thrice
VII: Countless millions
VIII: Many
IX: Many
X: Three or four
X-2: Three or four
XII: Once so far

[Edit] Dirge of Cerberus: Five or six

FFIV: Twice
FFVII: Thrice
FFVIII: Four Times
FFIX: Lost Count
FFX: Twice
FFX-2: Once
FFXII: Twice
FFT: Twice (Working on a third for the PSP)

I've played I-III, V, and VI, but I've not completed any of them. I lost interest in most of them.

FF VIII - had hard time to finish it once :p
FF IX - more than 5 times
FF X - more than 5 times
FF X2 - twice
FF XII - Twice
FF VII DoC once
FF CC on NGC twice

VIII - Almost once - haha
IX - Probably five times or more
X - Almost three times
XII - once

I've started VII and X-2 but in VII one i got bored and in X-2 i can't win the stupid Ixion.