How often and how much water you change depends on a lot of factors: Whether the tank has an established biological cycle, how many axolotls are in it, how much water you have in it, whether you have live plants, etc.
The best way to determine whether or not you need a water change is to test the water for ammonia, nitrite, and nitrate. If ammonia and nitrite are above 0ppm, and nitrate is above 40ppm, then you should do at least 20% water change.