I would let them get about 12-18 inches tall. Topping it once at 8 inches. If at 12 inches I feel the plant is very healthy and strong I would take it outside. Also don't put them outside unless there is at least 18 hours of daytime. Once the sun is 8 degrees past the horizon the light levels are low enough to be considered night. So sun rise to sun set may be 17.5 hours, but because of twilight you get 18 hours of light.