Here's the take from John Palmer's website.
"The lagering temperature and duration are affected by both the primary fermentation temperature and the yeast strain. These are the four primary factors that determine the final character of the beer. Some general guidelines for fermentation times and temperatures are listed below:
Check the yeast package information for recommended fermentation temperature(s).
The temperature difference between the primary phase and the lager phase should be roughly 10°F.Nominal lagering times are 3 - 4 weeks at 45°F, 5 - 6 weeks at 40°F, or 7 - 8 weeks at 35°F.
Stronger beers need to be lagered longer.
Nothing is absolute. Brewing is both a science and an art.
A common question is, "If the beer will lager faster at higher temperatures, why would anyone lager at the low temperature?" Two reasons: first, in the days before refrigeration when lager beers were developed, icehouses were the common storage method - it's tradition. Second, the colder lagering temperatures seem to produce a smoother beer than warmer temperatures. This would seem to be due to the additional precipitation and settling of extraneous proteins (like chill haze) and tannins that occur at lower temperatures."
I have got the best results from lagering as low as possible. YMMV