Well, that's a tricky one. Our first visit was in 1992 and we didn't go again until 1995. For us that was a great gap because we REALLY wanted to go again by then, were desperate in fact. However, we were taking our 3 teenagers (we always had to have two rooms ) and the whole holiday cost around £7,000 even then, so cost was something of an issue as well.
Nowadays, lots go every year or even more often. My feeling is though, that if you go for the second time too quickly, the magic will wear thin too soon.
I would wait until you feel you'll burst if you don't go and then hot foot it over there asap!!! Even though we now own at DVC, we still don't go every year, we found when we did go so often, it just wasn't as magical and we noticed all the broken things and lacklustre CM's etc, whereas going slightly less often, we don't notice them.