I did the Adv Dip first as I hadn't done any serious study before and it lead me into the BSc nicely, both as distance learning. I'm glad I did the Adv Dip first as the workload is lighter, obviously, and you'll be less likely to give in if you can't manage to dedicate enough time to it. The time depends totally on your ability to study, some on my course were able to do an essay in a few hours, I took considerably longer in the beginning as I did too much research.
Not knowing your circumstances I can only tell you what worked for me.