Aim for it least 5 times your tank volume circulation per hour. For example, a 55 gal tank would need it least 275 gallons per hour circulation. That is the bare minimum...10 times per hour is better and your tank will do better in the long run. With most soft corals, you can go up to 15 (better than 10) without major problems. 20 is pushing it for some as this gets to be too much.
My advice, get 4 smaller powerheads, put one in each corner and aim them all towards the same spot in the middle. This will create a good, random current in your tank. Corals do not like constant current from one direction all the time.