how long is a apprasial good for

Usually they are good for 90 days.

Per Freddie Mac Guidelines for conforming loans, see below:If the effective date of the appraisal is more than 120 days before the Note Date, the appraisal is outdated and a new appraisal with at least an exterior-only inspection is required. See Section 44.7(e) for additional information.Mike 703 505 5300

Depends on the mortgage type of loan you have. FHA is good for 12 months. Fannie Mae and Freddie Mac it varies