You can use the percentage tables in Pub. 946 to figure out your annual deduction. Keep records for next year. Be sure to read the section in Pub. If you are claiming depreciation on a property and dispose of it before the recovery period is over, you have to record the disposition and possibly recapture some depreciation as ordinary income.
Some instruments actually go up in value over time.
In this case see my article Depreciation for String Players for thoughts on how to treat this situation. Consider buying a good software program. It makes it much easier to keep track of depreciation over time. Also, if you find the whole thing confusing, consider hiring a professional tax preparer. A to Z of Musician's Deductions Musicians are allowed to deduct all expenses incurred to earn income.