Fermentation time for lager

Hi all, I brewed my first batch of lager last week, an Imperial Pilsner. I have it in a temperature controled lager refrigerator at 47 degrees (temp recommendation was 45-55 degrees for the wyeast strain). I made a yeast starter, stir plate and all so it was a healthy pitch. OG was 1.055. It is definitly krausening, 1-1.5 inches of foam on top and slow bubble rate in the airlock.

My question is: how aggresive should the krausening be? I've brewed plenty of ales and that stuff is usually rolling in the carboy, going crazy. Is that slow fermentation normal for a lager and how long does it usually take to finish fermentation?

Lagers are definitely slower! It's impossible to say for certain since every fermentation is different, but it should be finished with the bulk of fermentation at 10 days- two weeks or so. If you're doing a diacetyl rest, you want to do it when the fermentation is about 75% finished. After the diacetyl rest, the fermentation should be finished and the beer will be ready to back racked.

Yeah! The lower temperature makes the yeast eat more slowly. Sounds to me like it is doing just fine. The question for how long will depend entirely on the yeast. One way to tell how long to keep it in the primary is to watch the krausen. Take a piece of tape or something and mark the top of the krausen and check it every couple days. If it increases in size then move the tape. After a week or two you'll notice the krausen will start decreasing in height. At this point you should perform a diacetyl rest. Every day increase the temp by 2-5 (no more than 5) degrees F per day until you are 10 degrees higher than your primary temp. When you hit the 10 degree above mark, let it sit for 2 days and then reverse the temp using the same step method as before and take it all the way down to your lagering temp (which in your case should be between 33 and 37 degF). When you get down to your lagering temp, rack it to your lagering container and lager it for whatever time you decide.

Primary fermentation: 1-2 weeks
Diacetyl rest and step up/down: 3 days increase + 2 days rest + 6 days decrease = 11 days
Total fermentation (minus lagering time): 18 - 25 days