if there are no nutrients in the soil, 2 weeks is a good rule of thumb and if using synthetics go 1/4 to 1/2 strength first. fresh sprouts dont need fertilizer for at least 7-14 days.
In hydro you can put your sprouts in rockwool and start them in the lava rock/hydroton budding table. My table runs 1/6 of what the fert instructions reccomended for dirt all the time..