Started my PS in April and finished all my application in September 22 Just to make sure that if you are interested in Camb/Ox, get ready with your application about this time is neccessary (I wouldn't say it is a must)
Get your exams out of the way first and then start collecting ideas for it. Start jotting down reasons for wanting to study your subject as and when they come to you. Note down the names of books you're reading which are relevant. Try to get a draft done before the summer holidays and show it to your tutor, then you can work on it in the summer holidays. Your exam results may change your ideas a bit. Aim to send it off before the October half term if you can, but don't rush to be too early in case you change your mind when the reality of A2 starts to bite. You'll find quite a few threads from people who were in a rush to get theirs off and then found themselves regretting their choices and being forced to go through Extra when they changed their minds.