They made the "fox body" for 15 years, so it may be a few years for the 79-86, but I'd say at least 10-15 for 87-93 style because they're still very common and not much different from each other, IMO.
Man, they made so many Fox Mustangs, it's gonna be a loooooooong time before they get to be that valuable (although some people seem to think they are already)
Hard to say, but I do think they will be. When I'm in my 40's (19 now), I think they'll be getting like late '60s-early '70s Mustangs/Camaros/etc are now.
The Me too's will never be a classic car. No one will let them get that far out of use. From the strip to the street fighter, the Fox 3 is the Bomb for drag racing
it is gonna depend on the fox that you are refering to. no body cares about the 79's, most of them are beaten up drag cars now anyways (why would anyone want a 4cyl Pffff). I think finding a originall fox will be the hardest part. there will be a big market eventually to restore them back to originall. If in the 60's they had the same technology we have now I am sure they would have modified the **** out of theirs also!
SVO's, 84 GT's, 93 Cobra's etc are already getting to that status. the others will take awhile as they made so many.