Sam - we are the only school offering a helicopter-specific CPL(H) distance learning course. You can expect it to take somewhere between 4-5 months, based on 20 hours study a week - quicker or slower according to your personal circumstances.
There should be a big blue ad somewhere to the right of this page, but just in case there isn't, most of the questions you have should be answered at
Caledonian Advanced Pilot Training.
Please feel free to PM if you have any more questions
Phil