I would think 1 or 2 should be long enough. I had a AC110 on my 100g tank with 2 stems stacked on the intake. The intake was probably only 1/2 of the way down my tank.
I ran it like this for 24 hours to clear up the water from my sand while I was getting my tank set up. It was able to fully clear the water three different times after I mixed the sand (I had loaded it with quilt batting and rinsed it out each time I mixed the sand).