How often should i do water changes for my 25 litre aquarium?

Ok so i have a 25 liter aquarium with 7 guppies and 3 pearl danios. I feed them twice each day. My aquarium contains a good filer that can be used for up to 50 liters. Please do not tell me i have overstocked my aquarium and my fishes don't have a good life but they do they have a very good life in there. I normally change the water 25% every week.