The real estate market rate of change determines how quickly an appraisal becomes stale. In a fast-changing market an appraisal might be obsolete in less than six months. During a stagnant market values might not change for a year or more. That is a good reason not to order an appraisal until there is a need. Most institutional users of appraisal services have policies governing how old an appraisal can be before it is considered obsolete.