It took about two weeks to develop. I know that acet can form from bad sanitation, but I don't think this is it. I had have of the batch in a party pig and it was great for two weeks and only when it was almost gone did a little sourness appear. I read that oxygen at bottling can cause a reverse reaction of alcohol to acetaldehyde. I believe I let in too much O2 from my siphoning hose. I use brown bottles. It isn't skunk. It has a budweiseresque bite. It is still drinkable but I wondered if anyone else had this happen.