Every day. I adjust the input to keep the 20% volume. Plant size, Temp, RH maturity level all change the amount of nutrients that the plants use. In reality I am somewhere between 30% and 20% run-off each day. The extra is a bit of waste but it is better to error on the side of too much than the side of too little.
Ps. that is why I spent so much time perfecting MegaCrop it is cheap!