I'm Canadian, entering the USA for the requisite maximum of 6 months. How soon can I re-enter the USA after crossing back into Canada?
There is no formal limit on the frequency of visits or their cumulative length.
However you are not allowed to make the US your primary residence. If you start spending more time in the USA than outside it then the border guards may start suspecting that you are planning to make (or already have made) the USA your primary residence.
And the burden of proof is on the traveller, not the border guard. If you can't convince the border guard you are a legitimate visitor then you are likely to be denied entry.
In the most basic and common cases, meaning no Visa, Residency or any 'issues' with Immigration, you can spend maximum 182 days/6 months days over the previous 12 months.
So, if you enter the US and leave exactly on the 182nd day, you have to wait 6 months to enter again. That is the day your first day in the US is outside the 12 month window.
This is merely to be in the US legally. As noted, there are other considerations with different calculations.
"Travel by Land or Sea (including ferries): Canadian citizens traveling to the U.S. by land or sea are required to present one of the travel documents listed below, and may generally visit the U.S. for up to six months."
And a citation from the CBC:
How long can Canadians stay in the U.S.?
Usually a maximum of 182 days, or about six months during a 12-month period. Those days can be amassed during one trip or they could be the sum of several trips.
People from countries other than Canada are allowed to stay a maximum of 90 days.