There's no black and white answer. If and how long you are allowed into Canada is entirely up to the CBSA officer you encounter at the border each time you enter. If the officer feels you are visiting too often or for too long and attempting to live in Canada as a visitor, it's quite possible to only be given a short visit to Canada or even refused entry. This is even possible on the first visit if the CBSA officer thinks you may be more than a genuine tourist. Generally speaking, to avoid problems entering Canada, you want to make sure you are spending more time outside of Canada than inside of Canada in any given 12 month period. You also don't want to make it looks like you're moving based on what you're bringing with you and want to be able to demonstrate ties to the US if asked (e.g. job, property ownership / rental, etc.).
