I think for the date, just put an approximate date, they won't hold you to it. Also you can write a brief note on another sheet explaining that the date you return depends on when the processing is complete etc.
As for evidence, do you know where you will be staying when you return? With friends, family? Get them to write a letter saying you will stay with them until you find your own place. If you will be renting/buying, include print outs of your searches of properties. Perhaps communicate with a real estate agent and include some email correspondance. Do you have kids, will they need school? Will either of you be returning to school? Are you having your belongings shipped over to Canada? Show some quotes from moving companies or from storage companies if you are leaving things behind.
The website says 8 months but most cases seem to be completed in about 4. I made a spreadsheet of the London office (see below). Currently it is taking about 68 days for first stage approval (sponsor approval) Then another 2-4 months in London.
Join this thread, there are lots of people going through or who have been through the London office and will be able to answer any more questions you might have.
http://www.canadavisa.com/canada-immigration-discussion-board/thread-for-outland-london-uk-applications-t49436.2715.html