I find that without an ac in the same area as my plants my temps can get out of hand real fast even in a matter of 45-60 minutes....
My setup so you can compare...
(but many different factors play into your environment and ours may be different before your setup even exists)
SunSystem 250/400watt self ballasted MH/HPS Conversion Light with Hood
10" Vornado fan on the floor facing up toward the light
20" Box Fan on the floor, next to my table, facing straight forward and away from the plants to remove any heat surrounding my girls
8000 But window air conditioner in the room and in a window near the grow space and the air conditioner has a built in timer and thermostat. So when it gets to a certain temp in the overall room it shits itself off which helps a great deal.
Between the cool air from the ac, the fans and heat from the MH bulb my humidity stays between 30-35%