When I did straight coco fiber (husk can cause impaction) I changed substrate about every 2 weeks at that age.

You really need a false bottom so you have a drainage layer. I put down about 2" of false bottom, then a weed guard, and then my substrate. I also run a tube into the false bottom so I can drain it if the water gets around1" deep. I also kept a giant water container that i changed daily then filled back up with prime treated water.