You need to do water changes as often as it takes to keep your water from building up harmful elements.
This might be every week or every month. It depends on your stocking levels, how often you clean your filter, how often you vacuum, and whether you use undergravel filtration.
We can make guesses on how often you need to do water changes, and they might even be right. I can safely tell you that 25% every week should do the job, but that is probably excessive. Instead, do regular testing on your nitrates.
Choose a schedule that works for you. Let's just say for arguments sake 25% every two weeks, as this is pretty standard. Test your nitrates before each water change. If they are steadily rising each time then you need to increase the frequency of them. If they are the same or lower as the last time that you tested them then things are looking good.
Regardless of tests once per month should be the absolute maximum that you do them, as water changes also revitalize the water.
If you don't want to test for yourself then you should list your aquarium details including species of fish kept and their numbers, what filtration you are running, how often you vacuum the gravel, how often you clean the filter, and an idea of how much and how often you feed.
