There is no definite answer. Firstly, do you mean ''good'' or ''great''?? Because the thread title and the poll question differ at this. Secondly, what does ''good'' mean?? This is a very subjective thing, you know.
I've noticed that many Jonas Brothers fans consider their guitarist not good, not great, but awesome. So to these folks awesome means being able to play a few power chords and a bit more. Good then probably means being able to hold a pick and strum the open strings.
A really juvenile Metallica fan has slightly higher standards to call Kirk Hammett a guitar god. Every other guitarist that plays metal seems to be good and anyone not playing metal sucks and is a gay faggot. I've met such people, trust me.
A more mature rock fan would say someone like Jimmy Page or Slash is great. It's getting better. A jazz fan would have much higher standards and a stubborn classical guitarist wouldn't accept any effort but classical.
As far as I'm concerned, I try to refrain from phrases like ''great guitarist'', I rather say a great jazz or rock or blues guitarist. No one here would doubt that B. B. King is great, but no one would also doubt that B. would never be able to play flamenco or improvise over a series of altered chord changes. But B. plays blues and he is one of the best blues players that ever walked the earth, he needn't play flamenco or bebop. The same can be applied to anyone else.
That's why I voted for ''a few months''. You truly can become a great ... guitarist in a few months, be it punk or britpop or indie or grunge whatever of that kind.
I also agree with BrokenFighter that you can play for 70 years and still there'd be a place to improve. But also keep in mind that neither good or great means that you've learnt everything that can be learnt.