It depends for what purpose, and if you're ordering it on your own, or through a lender as a requirement for a refi or purchase loan. If you just want an appraisal for your personal reference, you can order it througn an appraiser and have it within a day or two.
However, if you're refinancing or getting a purchase loan, then the appraisal has to be ordered by the lender, and they have to follow the HVCC (Home Valuation Code of Conduct) guidelines. Lenders order the appraisal only after certain required disclosures are signed, and then the appraisal order has to go through an AMC (Appraisal Management Company). So in short, it can take a week or two to get an appraisal back, unless, as I mentioned before, you're just ordering one for your personal reference. Hope this helps!