The amount of time you spend per week is entirely up to you. It's possible to study full time, 40 hours a week or more, and it's possible to do just a few hours a week, or anything in between. Personally, I probably spent about 10-15 hours a week, although this is a bit of a guess because I didn't keep any records, and it took me 12 months to do all 14 subjects.
As to whether it's much more difficult than a residential course, I would imagine it is. Since most people have done one or the other, but not both, you'll probably find it hard to find realistic comparisons, but I would imagine that having other people around you to work with and discuss the subject matter with, as well as having a specific timetable where slowing down and never finishing isn't an option, would be huge benefits, and I would always recommend residential unless financial or time pressure means it's not an option. But I've heard of some people who are happier working at their own pace and prefer the distance learning - we're all different.
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