I have a bunch of G1 ponies I dyed, probably 10 years ago? They're all still in good shape and the one I painted symbols on hasn't bled at all. I used regular Rit dye and followed the directions for those.
On the other hand, I have some G3s that were dyed by someone else that haven't aged well at all. They're splotchy.
So it really depends on the pony and how you dye it. I think, going for darker colors on an already colored body is better than say trying to dye a white pony. At least this has been my dye experience.