How long should i wait till i change the water in my tank of betta fry?

About a month or more aince you can also add more water to the tank also

I change my water about every three weeks (I have multiple 20 and 29 gallon tanks and a 55 gallon)...It all depends on water conditions... The smaller the tank / bowl, the more often you will need to change it...

Have a good test kit and check for ammonia often...

I also check Ph... I use test strips which have multiple tests on them

Don't forget to add the water conditioners for the water changes... some tap water if very highly clorinated, depending on where you live...

Fry tanks should be getting daily water changes whether they are filtered or not. 10% to 20% daily water changes (definately adding dechlorinator such as Stress coat) will help the fry stay healthy and aid in growth.

whatever you do-do NOT separate the fry from each other or the tank as once they are-you cannot put them back together without fighting---only do a partial change and ake it about the same temp as the water you are taking out due to chance of ick developing due to change of temperature--