After the cycle is compleatly over.That is,when the ammonia and nitrates are at zero,and have been for a week or two.It also depends on the ammount of fish you have in the tank.If there is little or no load on the bio system,there will be little or nothing to skim. One clue that you need one is an oily looking film on the surface of the water.Another is when the nitrates seem to be rising steady.