Do you wan't to play electric/accoustic?
What sort of music you wan't to play classical, blues, rock?
If you wan't to play the electric guitar and modern rock (loads of power chords) it's going to be easier and quicker to learn than if you wan't to play the accoustic classically.
I'd say most people with lessons and practise could play a reasonable tune after 12 months, but as Jools says if you want to be good it takes forever as you never stop learning, people like Steve Vai, Joe Satriani etc practice for hours a day every day.
Most important thing to remember is even Eric Clapton didn't sound like Eric Clapton after 12 months so don't be dishartened if you don't either
Some inspiration fom my favorite
here and no I can't play like that
Darren