How long you are allowed into Canada is always up to the CBSA officer you encounter at the border. There is nothing you can apply for in advance that will guarantee entry or allow you to visit for longer than six months.
Visitors are quite often given six months - however it's entirely possible to be given only a shorter visit if the officer is concerned that you have plans to try to live in Canada as a visitor (which you're obviously not allowed to do). Generally speaking, to avoid problems, you want to make sure you spend more time outside of Canada in any given year than inside of Canada (this demonstrates that you really are just visiting). When you show up at the border, you want to make sure you look like you're visiting (i.e. don't bring more than a suitcase or so) - and definitely don't bring any household items or anything else someone wouldn't typically bring as a visitor. Also make sure you have a return ticket.
Once you're in Canada, you can apply to extend your stay. Whether your extension is approved is up to CIC. If you do end up getting an extension and staying a year (for example), you would probably then want to remain outside of Canada for a good 8-10 months after that before attempting to re-enter again to make sure it doesn't look like you're abusing your visitor privileges.
If you want to be able to legally live in Canada for more than six months of the year - they you would need to apply to immigrate and be approved as a permanent resident.
