It depends for some people yes, for some people no.
Slow-revisers like should have began then, but hey im a lazy **** and still havent revised for my upcoming mocks in 3 weeks. EEEEEEKKKKK!
My cousin however began revising at April, which everyone said was very late, but he came out with 5 A* 4A's and 2B's which is crazy considering he's quite lazy and often came to my house to chill, play ping pong, bash out UFC 2 or play footy almost every day.
However I have a friend who has been revising SINCE YEAR 10 I kid you not. Every day she makes nots, LAMINATES them, creates mind maps etc. but no doubt has she improved significantly-she's in the lower sets, however atm her current grades are good enough to beat some of the more intelligent ones in our sets.
I would say start after/during Christmas. Preferably start now.