You are on a type of visa even though you don't actually need to get one. It's a bilateral agreement between Canada and the US. You are not supposed to stay in Canada for more than 6 months out of the year as a visitor, otherwise you are considered to be living there so if the border guards figure out you are coming back after staying for 6 months, they may stop you and deny you entry. You probably would get away with it though but the proper procedure to stay longer is to apply to extend your visitor status, see
http://www.cic.gc.ca/english/information/applications/visitor.asp for the forms.