I have a total of 21 exams, and have been revising on and off since december, (as i had mocks for all my subjects), however, have started doing more serious revision lately.
I have made myself a revision timetable but because i struggle to keep to times i set for myself, as long as i revise for the subjects that i have set for myself, the time i start doesnt really matter. (I try to do about 3 hours a day and much more on the weekends, but thats easier said than done)
Although, its hard, i would say an important thing to do at this time is to not compare yourself to any other people who are doing better because that will lower your self-esteem down, everyone is capable to a certain extent and has abilities in different areas.
There isnt a specific time to start revising, but obviously the earlier the better. Make sure you have regular breaks whilst revising as that will help you to concentrate and increase your focus.