I would guess that most people do a 20%
pwc about once a week. (Take out 20% of the water, then add that amount back in from your tap.) If your tap water has chlorine in it you will need to add a dechlorinator every time you change your water. (I'm guessing this is the "tap water stuff" you asked about.) A lot of city water also has chloramine in it, so make sure your dechlorinator takes care of this, too. (I believe most do.) I personally do a gravel vac every time I change my water, vacuuming a different part of the tank each time, so that in a month I have covered the entire tank at least once.
This is pretty general advice. What really determines how often you should do a
pwc is the level of nitrates in your water. High levels of nitrates are harmful to your fish, and by changing part of the water you reduce the amount of nitrates in the tank. Anything over 40
ppm is considered harmful. I personally try to keep my tank under 20
ppm. If you are unfamiliar with nitrates, check out the
nitrogen cycle article here or
this article.
Your tank looks very nice! I especially like the pinkish-red rock right in the middle and the wood to the right of it.