the place we got my ring has free dipping for life, and they said not to do it more than once a year if you can help it because in order to re-dip it they first have to buff it up (almost like mini-sandblasting haha) to prepare the surface for the rhodium to stick to it. so if you do it too much, your band will actually wear down over time.
we got my ring in october of 2007, i had it dipped right before the wedding in july 08, and have not had it done again yet. it was pretty yellow in comparison to my wedding band back last year, but now they both look the same next to each other. ill probably get it done every year using our anniversary as a reminder
oh and it takes less than an hour at my place.