How long the appraisal takes can be dependent upon how busy the appraiser is. My husband (real estate appraiser) can turn out an appraisal in the matter of a day or so when he's not busy. Other times it takes a couple of weeks because you are in line behind a number of other buyers, homeowners refinancing, etc.
Speed can also depend upon the condition of the property. It can depend upon the similarity or the uniqueness of the property. Is it easy to find comparables, or very difficult?
Also, different loan types (FHA, USDA, VA, etc.) have different appraisal requirements.