The new filter in the 2005 Sundance's is pretty expensive, probably too expensive to replace every couple of months ($80?). I thougt someone else posting on the board said the filter was rated for 8 or 9 months, IIRC.
The three month interval is what we shoot for, you will be able to tell after a while when it's time to change the water. You might not even get 3 months out of your first fill. Spa use is pretty high in the beginning, coupled with the new owners tendency to overmanage their water.
Brewman