I agree with the other posters. I am a native speaker, so my situation is somewhat different and I can fully understand you wanting to take more time to review English and become proficient.
That being said, however, I think 3-4 months of focused intensive study is what you should shoot for (in terms of GMAT-specific study, not just English as a whole).
I studied around 2-3 hours/weekday and 5-6 hours per weekend day for 4 months, and let me tell you, when you get to around week 13-16 of a regimen like that, it becomes very difficult to maintain focus and keep forward momentum. The first 6-8 weeks are great, and you can see big results and are pleased with your progress. But around week 10-11 you'll hit a serious plateau, likely in both areas of study (math/verbal). After that, it becomes a will-yourself-to-the-end grind in order to eke out even the smallest amount of improvement.
Combined with all the practice CATs, not to mention classes (if you're taking a class), your life BECOMES the GMAT. And I can't imagine that lasting for 12+ months.
I have heard some pretty crazy stories of applicants in China with absolutely insane work ethics in GMAT study, non-English speaking kids who study 3 hours a night for 4 straight years and end up rocking a 790 on the GMAT simply due to such an overwhelming body of work and time put in. Hats off to them - there's no way I could do that.