Presuming you are sowing seeds or plug plants, just keep it damp, never let it dry out and don't water-log it.
If you are growing on small plants, it pays to let the tray stand in water until you see the top of the soil getting damp. Welcome to GOY btw.

You can't rule on when or how often because it depends on the conditions - obviously in a warm dry atmosphere more frequent watering will be needed. Drain the water out of the tray when the compost is thoroughly moist though as too much water is as bad as too little.