The prizm takes some tweeking. Start with the water valve closed and make minute measures to open it, testing to see the desired effect. What you want is to get dry foam rising up the neck of the collection cup and spilling over into the cup itself; you don't want your collection cup to fill up with seawater, however...which is where the "tweeking" comes in.
How much skimmate you get will depend on the bio-load of your system. Basically you should clean up the collection cup at least weekly, or, if you get a lot of skim, then more often.
The time for break-in varies. Perhaps as long as a week; but, if you're lucking, you'll get it running right away within a day or so.
Remember that with the prizm you can't empty the collection cup with the water line above the O-ring, otherwise you'll get seawater spillage off the backside of your tank. You either need to turn off the skimmer at the power source, or turn down the water intake using the valve. If you use the valve, then you will have to reset it agian.
It would be nice if the prizm came with a handy on-off switch, but, alas, it does not.