I follow the week for 2 Plato rule, often longer. So that is 6 weeks for a 12 Plato beer, which is 1.048.
My low gravity lagers are usually done with the D-rest in about 6 days. Pitch at 46F, ferment at 49-50F, then the D-rest. Then you can crash it down, and I was at 32F last year, and will look at 30F (well -1C) this year. Check the presentation at the 2012 NHC by the Chuckanut guy on brewing lagers for what cold lagering temps do.