Fermentation is a function of temperature, and all yeast have different temperatures, but they are all pretty much between 68 and 75F. The lower you go, the slower it will ferment, to a point. Then after that, they will die off. As you probably know, you'll need a lager yeast to get down into the 50 and 40's. I wouldn't go below 68. Once in the secondary, or clearing, carboy, temperature really doesn't matter, it's just used for clearing the brew.