You do NOT need one for Canada, no matter how long you've lived here. CIC will specifically request one, only if it's required.
Police certificates
You and your family members who are 18 years of age and older and are not permanent residents or Canadian citizens must provide:
valid police certificate, or
police clearance, or
record of no information.
These documents are to be provided for each country other than Canada, in which you have lived for six (6) consecutive months or longer since reaching the age of 18.