Are there powerheads in the tank, or is this pump your only source of flow? On my tanks, I aim for 5-10 x turnover through the sump and 30-40 x turnover from powerheads in the display.
This is not exact, in fact could be quite a bit off, but I figure with a 660gph pump, if you divide that by the number of levels on the controller, 20, you get 33gph. So with 33 per level, at level 7 you would be slightly over 10 x turnover, not counting head loss going up to the display. And with not counting head loss, at level 15, you're theoretically sitting at 22.5 times your display tank volume.
If you want to be more precise, you could see how long it takes to fill a gallon jug and do some math based on that.