If you're a regular reader of this blog, then you'll know by now that I'm not a fan of the word "good." Why? It's a judgement. Judgements require an immense amount of energy to maintain. I personally feel that judgements get in the way of learning guitar. To me, the process of becoming familiar with how to play guitar is far more important than becoming good.
That said, I think it's fair to address your question of how long it will take to be good at guitar. There's no point in you doing this unless you have an end point to shoot for. If your end goal isn't defined, if you can't define what "good" is, how will you know when you're there? Let's throw some ideas around.