I recommend leaving the beer on the yeast as long as they are fermenting sugars. Once they stop, let them rest for 2-3 days and get them off so they don't start dying and spilling their guts in your beer. Monitor your airlock and when it slows down to a trickle, take a gravity reading. Then wait a day or two and take a second reading. When you get two readings the same, the beer can be racked off the yeast at that point.