The time needed to prepare for the GATE exam varies greatly depending on individual factors like study habits, whether the concepts are clear or not and etc...However, a timeframe of 1 to 2 years is generally recommended for serious preparation, especially if you're aiming for a good rank.

If your basics are clear and you are well known with the concepts then focus on pyq's and give mock tests and re-revise the syllabus. But if your preparation is not that much good then I would suggest that give a complete one year time and prepare well and then appear for the Exam.