How often should a violin be re-appraised to account for the increase in valuie?

The re-appraising time line depends on several factors, including the price range that the instrument is in and market conditions. When the economy slows down the market conditions typically slow down and values don't rise as quickly as they do in a good economy. If you have a "fine" instrument, I'd check with your appraiser every couple of years to find out if it is worth having your appraisal updated. Most appraisers will, for insurance purposes, appraise on the high side in order to give you a few years between updates.

One thing to keep in mind is that student instruments typically depreciate with use and wear and tear. Many students falsely think that their instrument should go up in value, when in fact, they usually don't.

Disclaimer: Every appraiser has their own thoughts on value and the rate at which it increases.