it all depends on your tank size, for tanks 5-20 gallon you need to do a complete water change every week cleaning gravel and all decorations each time
for 20g and larger it is recommended that a minimum of 25% at least every other week by using a vacuum system you can clean substrate as you remove water
a good site to learn more is freshaquarium.about,com/od/fishqa/fishqa/f/faq0110.httm
