If memory serves, I believe that is is in the first 48-72 hours of pitching yeast. This is when they are most active in producing phenols, fusels, esters, etc., and if you want to minimize those, make sure to keep a 'leash' on the yeast by regulating the fermentation temp. I do this with a temperature controller probe insulated with styrofoam, duct-taped to the outside of the fermenter.
I ask the question because I only have room for two vessels in my fermentation chamber/lagering fridge. Currently, I have a California Common in there, and the yeast was pitched Saturday night @ 9pm (pitching + 84 hours at the moment) at 64 degrees. The yeast is actually Wyeast 2204 Bavarian lager.
I have a bit of an aggressive brewing schedule coming up. I am planning on brewing a hoppy american wheat tonight, THEN I am brewing an Oktberfest with my wife on Sunday. Bottom line, I need to have the Oktoberfest set to 50 degrees on Sunday, meaning that the ferm fridge will likely have a lower ambient temp.
My current plan is to brew the wheat tonight (estimated OG of 1.051), pitch a stepped-up US-05 starter (as I need a quick and vigorous fermentation, this wheat is going to be kegged in 12 days) at 66 degrees, then on Sunday (pitching + 4 days), remove the wheat, leave it in the basement (ambient temp of ~74 degrees) to finish up.
The only issue with this plan is that I would need to remove the California Common tonight, as if I tape the probe to the wheat, the freezer will likely get down really low to try to stabilize the fermenter temp AND I would need to do the same with the wheat to make room for the O-fest.
Clear as mud? I'm just trying to minimize unwanted esters, phenols, fusels, etc., and was hoping that I could let these fermentations go after a few days. Alternatively, I could rig up the <shudder> swamp chiller.