So I had this question going around in my head for a while.
And if you use an NVIDIA card, the question can be answered pretty fast. Just use the CLI tool nvidia-smi:
So with the current ETH DAG #131 you can comfortably mine until April 2018, when the expected DAG size would exceed the RAM of a 3GB GPU.
It’s expected that the ETH network will switch to a “proof of stake” algorithm by then, more here. So don’t waste your money on 8GB cards.