Outside of this site, I have tutored many students on IELTS preparation and I get asked this question a lot. My answer is always the same: there is NO time frame that I can give you. This is because every student is different. Some students put more effort into the learning experience, others not so much. Also, other factors to consider include your "current" level of proficiency, how many hours and days a week you book lessons (and learn with a tutor), and how often you practice on your own.