I'm currently considering the purchase of a few Antminer S9 13.5 THs and I'd like to have an idea of how long I could (or should) plan to use one before replacing it with a newer model.
I've seen some forums suggest 8-12 months, while others suggest it might be as long as 2 years.
Clearly, a part of the answer is based how long I can squeeze a profit out of each machine. In terms of major costs, my local electricity cost is $USD 0.08 / kWh and I will purchase the units directly from the manufacturer (so around $USD 2,320 + tx + shipping).
I understand there are many more variables, like how much faster future miners become, how fast the difficulty rate increases, BTC pricing etc, but I'd appreciate any general guidelines or wisdom you can share. Cheers!