If you are genuinely committed to working for the exams for an an average of five days a week and six hours plus per day, then you will be putting in the same amount of time as a full-time residential student.
In this case, the following may be a helpful guide. How you apply it depends on your own circumstances.
At Oxford, our Phase 1 residential modular groundschool lasts 14 weeks. Our Phase 2 lasts 12 weeks. The students have 4 weeks off in between, to sit the Phase 1 exams and get their results in order to make any necessary decisions (depending on their results) before starting Phase 2, and also to have a much-needed break.
These students have the advantage of full-time face-to-face access to instructors and they also give each other mutual support. They often explain things to one other, both during the day and in the evening. As a distance learning student you will be on your own and your access to instructors will be confined to phone and e-mail, apart from brush-up courses.
I think that you would need to be quite exceptionally capable and committed to do it in less than the 30 weeks that our residential modular students take.