It is do able in 4-6 months as long as thats all you do and nothing else apart from work.
I started the same time as a pilot I knew doing full time ground school at OAT. He was a bit suprised when he saw me at a brush up course when he was half way through. And he was positively pissed off when I saw him again before he had completed the first set and I was doing the final 7.