Mock tests are where preparation becomes exam performance. The key is not only to take them, but to review them in a disciplined way that changes what you do next.
Usually after you have a workable first-pass foundation and can tolerate mixed-subject testing without it becoming random noise.
Frequency should increase as the exam approaches. Early on, spaced mocks work well. In the final phase, many students benefit from more frequent full-length simulation.
Review. Mock tests become powerful only when you analyse wrong answers, time loss, and weak subjects in detail.
The best mocks are not the ones you finish. They are the ones that change your next revision cycle.