Is there a best practice or industry average for how long spare parts are in a back order status?
There is no standard for something like this. If the spare part is a unique item that is hard to manufacture, it can take as long as a year. However, 3 years is extreme. Get with your contracting officer to cancel the buy and come up with other options. Talk to your item manager at the primary inventory control, probably one of the ALCs and see what your options are.