I did a 60 point course in a year (so that translates to 10 hours a week compared to 5 over two years if you see what I mean). I think that is enough time. I did about 5 hours a week - but I am one of those people who cram!
Get in touch with the OU and ask them how many hours a week they estimate you will have to dedicate to study. I applied to do an MA in English Lit. with the OU and they reckoned for that particular course I needed to set aside 14 hours a week for study. Bear in mind that if you need to use libraries etc. you will need to factor in travelling time too.
Btw, I got cold feet and pulled out. Mainly because I found I out I was pregnant with my second child and suddenly 14 hours a week sounded like a hell of a lot! But Good Luck.