"3 to 4 months" according to the original article - I don't know what the usual thing to allow casual visits by Canadians to the US is, but I travel there from Australia fairly regularly for vacation and visiting family and friends under the visa waiver program, and I know I'm limited to 90 days - if I want to stay longer than that I need to get a visa.
edit: Too add that especially after 9/11 and all the border fervor, it unreasonable to not be prepared when crossing.