Depending on whom you ask, it can be anywhere from 1 week to 2 weeks at that temperature before lowering the beer down to lager temps.. Palmer advises to ferment lagers in glass carboys so you can tell just when the krausen has peaked and started falling back into the solution. He also recommends the lagering temperatures be no lower than 10 degrees below the fermentation temp. That means, you'll need to lager your beer at 50f for about three to four weeks time.
You're shooting for about 3/4 worth of completed fermentation before you start to lower the temperature on the beer and lager it. The colder the fermentation, the longer the lagering period. Higher gravity beers take longer to lager than sessionable beers.