So, are you actually lagering the beer? Typically, brewers will pitch lager yeast into a beer and allow it to ferment around 16-18c for a week or two, then lager the beer at about 10c for a minimum of two more weeks, but more typically six to eight weeks. If you're not taking that lagering step, then you're treating the lager yeast like an ale yeast.
If you want a faster, high attenuation ferment on your beer, you can switch to an ale yeast. Even then, if you aren't using a hydrometer to monitor when the actual fermentation is complete, it'll be hard to judge when to bottle the beer. You would probably be safe bottling your beer at three weeks in the primary fermenter for most low to mid gravity beers, but i'd go a month or more on high gravity beers with LOTS of fermentables.