Hi,
How big is your tank? My tank is small, 5 1/2 gallons, and it holds 4 1/2 gallons of water. I vacuum 1/2 of the gravel (just one side of the tank) once a week, and change 1 gallon of water. That's about 20% for my tank.
The next week, I'll vacuum the other side, and change 1 gallon of water again. This keeps my nitrates at an acceptable level. This is just regular cleaning and maintenance. If your tank is finished cycling, this is probably all you need to do. If your tank is still cycling, you need to do your tests every day and change your water based on what the test says.
If your tank is bigger, you may not need to change some water or sweep every week. Also, see how much waste you are vacuuming out. If you see a lot, you may have to sweep a little more frequently (and check your ammonia too if you're seeing a lot of waste). Do your water tests so that you can see when the nitrates start to rise. When they rise above 40ppm, then you should change the water. Then you could do a 20% change to bring them down. They may not rise as quickly in a big tank as in my small tank. You could get away with every 2 or 3 weeks, maybe. Just do your tests and keep an eye on your parameters. I hope this helped!