If you are entirely a beginner, 10-12 months of advance preparation before Prelims is usually advised to cover all topics for Prelims and Mains. This means, if you are planning to appear for Prelims 2020 (which is on June 2020), the recommended time to start preparing is June – August 2020. However, there are many candidates who can clear the IAS exam successfully with 5-6 months (or even less) months of advance preparation as well. Thus the key point here is not the quantity of time, but the quality of time spent. Aspirants should integrate Mains preparation with Prelims preparation, as you won’t get enough time to cover all Mains topics after Prelims (only 3.5 months). For efficient, fast, and focused preparation, candidates are advised to take ClearIAS Prelims Mock Test Series 2020 as per the timetable.
https://blog.oureducation.in/best-ias-coaching-institutes-in-goa/