How long can a us citizen stay in canada?

I believe that it depends how long your Visa has set for you.

90 days. And don't even dream of overstaying that. You will be found out and deported, never to be allowed to enter Canada again. To immigrate to Canada, start here: http://www.cic.gc.ca/english/immigrate/index.asp Note, you cannot under any circumstances apply for an Immigrant visa from inside Canada. You must apply from your home country. Don't waste your money on immigration consultants, they can do nothing more, nor faster, than this site allows you. Fill in every single sheet and all details BEFORE you submit your application. If you have to wait for some forms, wait. Every question, every extra document that you submit will delay your visa at least one month. You are dealing with a bureaucracy here. Normal visa processing time is 18 months to five years. Don't count on your boyfriend to sponsor you, Canada immigration takes an extremely dim view of relationships which haven't been well established for years, like being married five years. There is no faster way for Americans than for anyone else from anywhere in the world. All visa applications are treated equally on a point system based on education, language, diplomas, skill level and profession, and more. All of that should, technically speed up your application but don't on anything very fast. Remember, don't overstay you visitor visa. You will get into trouble, far more than you bargained for. You cannot work in Canada nor open a bank account or get a credit card without a SIN (Social Insurance Number) which you can only get after you have your immigration visa. Sorry to rain on your parade. Immigration to another country is a serious matter, not to be taken lightly.

I understand that the maximum stay is 1 day less than 6 months

The better question is why anyone would want to do that.