IIT JAM Mock Test Papers are the best choice for the practice because it gives you a clear idea of managing time and approaching paper effectively. If you know how to approach the paper, then 50% work is already done. More practice always leads to making fewer mistakes in the real examination.
you should start solving IIT JAM Mock Test Papers at least 2 months before the exam. Solve at least two mock paper every week (for one month). and, 3-4 Mock Test Papers per week in the second month.
there are many sites that provideIIT JAM Mock TestSeries. You will get the mock test series in this link.