I've replayed almost every Final Fantasy that I've ever played with the exception of Final Fantasy V which I only recently finished. Although I do replay the games on a regular basis I know what you mean, this is because sometimes I do stop, sometimes I go all the way through, I find that if I stop playing the game for something else, I won't want to come back to it. For instance, if I replay Final Fantasy VII, everything will be fine and I'll be interested the whole time unless at some point I decide, "I feel like playing Final Fantasy IX" Even if I only play the other game for a day and then go back to VII I'll find myself totally uninterested and usually end up deleting the save game.

Although I've played through some Final Fantasies so many times I don't even know what to do with myself... (e.g. I think I've literally played through Final Fantasy VI over 10-12 times)

I'm a bad example though because I am utterly obsessed with Square games, anything I say is immediately biased and unrepresentative.