How much detail on my extra curricular activities should I include in my Oxbridge personal statement?

This is one of the areas in which Oxbridge most differs from other Russel Group universities. You should only mention extra curricular activities if they are directly relevant to the subject you want to study, for example, if you are applying for History and you have done work experience at your local museum or archive. If your extra curricular activity isn't relevant to your subject, it's best just to leave it out - Oxbridge tutors are much more interested in your academic potential, so unless you have something highly pertinant it's better to leave out extra curricular interests entirely and use those words to talk about the reading you've done instead!