There is one US law limiting your stay and both the US and Canada have some legislation which has potentially undesired consequences if you spend a lot of time in the USA.
Number one on the US side is immigration. You are admitted as a visitor and may stay for up to six months in one visit. Repeat visits are at the discretion of the border officer. There is no written rule like the Schengen zone in the EU has for how many days you can spend in a given time period in the USA. We have a letter from the CBP on this site claiming their rule of thumb is 91 days after a 90 day stay but it's unclear whether they apply that rule to Canadians (90 days is an ESTA limit) and what's the rule of thumb for shorter stays.
Then you need to consider US tax. There is a Substantial Presence Test but also Conditions for a Closer Connection to a Foreign Country which says:
Even if you meet the substantial presence test, you can still be treated as a nonresident alien if you:
Are present in the United States for less than 183 days during the year,
Maintain a tax home in a foreign country during the year [...]
Have a closer connection during the year to one foreign country in which you have a tax home than to the United States
Now on to the Canadian rule: All provinces, except Ontario and Newfoundland, require you to actually live in your home province for at least six months plus a day (183 days in most years) in order to be considered a permanent resident of that province, and therefore qualified for provincial health insurance (medicare) benefits. Ontario allows you to be out of the country for 212 days (seven months) and Newfoundland for eight months without risking loss of your medicare benefits.
Thus most Canadians do not wish to stay more than 183 days in the USA a year making the immigration limitations quite a bit moot.
Finally, a country always admits its citizens at the borders. That is the very meaning of being a citizen.(If they are capable of proving they are citizens indeed and the airline allowed them to board -- neither is a problem while you have a valid passport, the fun starts after. For the Canada-US border you do not need to fly, you could drive or even Crossing from Canada to the USA without a passport on public transit?)