First of all seconds. I built it that way because it was supposed to be used for large counts, but I agree that 10 seconds is arbitrary, could expand that.
I don't think that it's a good idea to move all these clouds across the complete screen. Make the spawn area a rectangle, move everything a bit to right, fade them in and out. That should be enough for the effect and you don't need to have 60 seconds warmup times to look good.