The burden of proof that the Canadian citizen is not an intended immigrant (plans to make the U.S. their primary residence) is always on the applicant. There is no set period of time Canadians must wait to reenter the U.S. after the end of their stay, but if it appears to the CBP Officer that the person applying for entry is spending more time over-all in the U.S. than in Canada, it will be up to the traveler to prove to the officer that they are not de-facto U.S. residents. One of the ways to do this is demonstrate significant ties to their home country, including proof of employment, residency, etc.