How often is a diacetyl rest necessary? Also, if I were to take the fermenter out of the chest freezer for the diacetyl rest, does it matter much if room temperature is 65 or 75?
I suppose that kind of sounds like a stupid question...
Definitely not a stupid question. As stated, the best thing to do is taste the beer and see for yourself. You don't need to be a Master BJCP Judge to identify it. Think "I Can't Believe it's Not Butter". While some strains seem to produce more than others, I have found it is almost completely mitigated by pitching the proper amount of yeast, at temps below the fermentation temp, and letting it rise to somewhere a few degrees below the middle ground for the strain. I typically shoot for 48F-50F, starting at 45F.
As for the temp of the diacetyl rest, I have done mine at 68F, mainly because that is the temp of my basement which makes life easy. Would 75F hurt your beer? I think not so long as it is fermented out at around 80% or so. Like Denny says, the purpose is to make the yeast more active to consume the diacetyl. Now if you got into the 80's, I would suspect the yeast would start to produce some off-flavors but maybe someone else can provide a more educated response on that......
Dave