It's highly variable depending on the RH, lights, how fast they grow, nutrient levels, ...
The best guide is to go either by feel near the surface, or to go by the weight of the pot. Get a feel for the weight of the pot when dry vs after watering. The difference should be dramatic, you should be able to judge how dry they are by weight after you get yourself calibrated by feeling wet vs dry weight. Let them get fairly dry/light-weight before watering until some water comes out the bottom. Don't water again until they are fairly dry/light-weight again.