If I buy a mining rig of 6 x Radeon RX 580 8GB cards and start using it 24 hours a day, 365 days a year with the temperature below 70 degrees Celsius. How much will the rig depreciate in value every year?
I've been GPU mining for over a year with multiple rigs using a variety of cards from RX 470s to GTX 1070ti.
I have not noticed a depreciation in performance over the year. Performance in this case is measured by outputed hash rate, watts used and up time.
Each of those three parameters have stayed constant over the past year.
Now if by depreciation you mean what you can sell your GPUs for in a year - well that depends on the market. New GPUs are selling for more than the MSRP of a few months ago. Used GPUs are selling for a premium. What will the market be in 12 months? Don't know.