I would give those seedlings 150 - 200ppm nutes to start. There is a balance where the plant will use as much nutes as it does water, so your water level will drop as it soaks up water, but your nute PPM will stay the same. If your water level drops and your PPM goes up, you have more nutes than it can eat. If your PPM is dropping, it's taking more nutes out of the water than it can drink. So, every day more or less check your PPM, ph, and water level... keep your res temps under control.
I change my buckets once a month pretty much just to clean everything down. If you stay on top of things in your rez and maintain your solution, there is no need to change your water.