In most cases, appraisals are valid for only 120-180 days. Inside of a year, the appraiser can go out, reinspect the property, and recertify the original appraisal.
They need to be limited in time, because values change over time. In some markets, values have dropped 15-30% since last year. The banks need to know that, and so do you, since you really don't want to end up financing more than your home is worth.
A word of caution. Many lenders doing HELOC's will only use an AVM, automated valuation modules. Computerized appraisal. It's nice, because they're super-cheap, and the bank will likely not even charge you for it. But, they aren't always terribly accurate, and you could end up upside down. That happened to someone on this forum recently, upside down by about $45 grand. OUCH. Better to pay $350 and ensure you're dealing with accurate numbers.
10 years in mortgage banking