If growing from seed...you have 2 opitons.'1. vegging takes a bout 2 months...so you can change to 12/12 at 2 months...or;
2. some people have space issues and flower their plants between 4-6 weeks or when the plant is 12-18" tall...plant will grow another 100-150% after 12/12 change.

At rez change...mix nutes...top off when necessary(no nutes) in between rez change with ph'd (5.8) water, and you could add superthrive, 1/4 tsp gallon