In order for an appraisal to remain accurate, it must efficiently reflect the current market. Whenever the appraisal is desired for an accurate point of reference, it must be able to show the present state of the market. While there is not an exact expiration date on appraisals, many lenders will only accept an appraisal report that is about 60-90 days old. After that, there are most likely sufficient changes in the market that require an adjustment in the report. If you need a change within this time frame, many appraisers will make adjustment for a small fee.
If an extension is needed on the report, some lenders will allow it with the proper paperwork and documentation. However, in a market that is noticeably declining the time in which the report is valid becomes significantly shorter.
The maximum period of validity for an appraisal is six months, as the market will definitely have changed and it will take more than minor changes on a report to reflect this.