Bribo is right on the money, as usual. lol
Fish only can be higher than if you have corals. If you keep corals I;d say you wills ee some change in them if you go higher than 20-30. With fish its harder to see a change but I'd say dont go over 30 or so. If its not a regular thing to miss a water change you can always do a larger one the next time you do one.
If you keep any SPS
corals you will probably notice a big change if nitrates are over 10 or so.
I tend to do large water changes on my tanks, some every 2 weeks and some every month. I change about 50% regardless.