Nice, that's a very decent annual return for the risk.5 months minimum with current gfx card prices if ETH value stays around $3900. If you're paying normal retail price for cards then longer. I paid my rig off in under 3 months because the ETH I was holding doubled in value. I sold my first few ETH last night.
How are you dealing with tax? revenue or capital gains and can you depreciate/write off the rig?