I am sharing my complete strategy. I can assure you 6 month of serious & smart study is good enough to clear IES.
My answer is going to be a bit longer, but I am sure it is very useful.
Let me confess that my result (AIR-5) is way more than my work/time/energy spent in preparation. I am working in NTPC ltd (6 working days per week/no weekend)& I am a fitness freak too(usually spend more than a one/two hr everyday in physical activity), bottom line is “the time available for me was very scarce”. No need to say that I have availed any coaching neither joined any test series. The only external help I got is free interview guidance from ACE Engg Academy(bcaz it was for free) & experience/guidance shared in IES helpline website (https://ieshelpline.wordpress.com/)and IES made easy classroom notes. I don’t have any friend to discuss any topics or clarification of doubt (which I really missed).