There is no set period of time Canadians must wait to reenter the U.S.
But...
If it appears to the {US} CBP Officer that the person applying for entry is spending more time over-all in the U.S. than in Canada, it will be up to the traveler to prove to the officer that they are not de-facto U.S. residents.
CBP Info, see the last paragraph.
OTH, we are allowed to spend there no more than 182 days in 12 months period. They count days, not weeks or months. Which means - any number of days out is counted as time out. It also means
that you can't enter the US again even for ONE day, unless you stayed more than 183 days "out" in the last 12 months.