A lot of factors effect a 221g. First of all the incoming mail is checked and security screened, then it needs to be sorted and assigned to the correct section, the person responsible for your case needs to check the documents, and then a consular Officer needs to verify the data and adjudicate your visa.
it is also very dependent on your assigned post and the workload that post has. It can take 2-3 weeks, it can take 2-3 months. As processing varies in each and every case they cannot guarantee an outcome on or before any given date. This is why all consular posts, as well as this site suggests you DO NOT purchase any travel plans until the visa and your passport are in your possession.
