The amount of time spent studying differs from person to person. However, the highest scorers who are competing for the first time have admitted to dedicating no more than three months to it. However, it will have to consider your study rate as well as your ability to grasp the subject and recall facts and numbers in both general studies. Taking all of these into consideration, it is recommended that you go over the full prelims study material at least once before committing to three months of real-time preparation for the Prelims.
It is up to you to decide how much time you should devote to the prelims based on the plan you have implemented. If you’ve been studying for this exam since college, there’s a good chance you’ll have more time to devote to prelims without worrying about the Mains.