If it is not cycled then twice a week(Wed and Sat for example), 50%. Onc eit stops getting cloudy you can drop down to 25% once a week. That is, as long as you just have a Betta in there. It is a good rule of thumb to do 25% if you have one fish, about 30% if you have three, 50% if you have more then 3. Once it is fully established 25% a week no matte rhow many fish you have
In a filtered 10 gallon that is cycled 25% a week is fine, especially if you vacuum the gravel every time. If you keep a tank with alot of plants then you only want to do 25% of the gravel each week too and rotate the tank section you do weekly.