Funny.. I read this as biotransformation widow. What that is I do not know.
The window for yeast and biotransformation however seems to be during active fermentation before the yeast starts to "clean up". It will vary by yeast strain. For something like Nottingham, I'll add dry hops 1-2 days into fermentation, for something like the kviek yeasts, I'll shoot for ~when I pitch the yeast if I am looking for that hazy, hoppy stuff..