There's no black and white rule - that's why you can't find the answer. When you enter Canada immigration is looking to see that you are entering Canada as a genuine tourist and have no plans to remain in Canada long term. If anything about you (how you act, how you talk, what you packed, how recently you visited, how long you plan on staying, etc.) says otherwise - that's when you run into problems. There's no one thing that determines how long and if you will be allowed into Canada.
So US citizens can stay in Canada as long as they want, but every six months they have to pass an arbitrary test of honor where their ultimate fate is in the hands of an unpredictable immigration officer.
Honestly, that's quite a bit better than the conditions of any other country I've visited / overstayed my visa in