The on-target weekly study hours for the Oxford distance learning package is 15. This is done during 18 modules for phase 1, then 19 for phase 2, scheduled at 1 week per module.
Plus there are 2 lots of 2 weeks onsite brushup courses, plus as good as 2 weeks worth of exams.
This is quite a lot if you are doing 55-60 hours per week at work - you also need a fairly clear head for a lot of the material.
Having said that I did 70%+ of my phase 1 studying on the train to and from London each day, and managed to achieve good marks.
Good luck whichever way you choose to do it.