The general rule is a maximum 180 days stay in the US as a Canadian, but you may be suprised how the IRS and border agents calculate your stay, see below. The Canadian Snow bird association has been actively working on this issue.
The bipartisan "Visit USA Act" infront of the Senate and in the House of Representatives, includes a proposal for a “Canadian Retiree Visa".
"The act would include a new“Canadian Retiree Visa” that allows Canadians over the age of 50 (with derivative benefits to a spouse and minor children) to have a visa which lasts for 240 days, and is renewable every three years.Canadians wishing to take advantage of this new visa would have to be able to demonstrate that they own a residence in the United States or have purchased rental or hotel accommodations in the United States for the duration of their stay.They would not be allowed to work, nor would they be eligible for any form of U.S.government assistance or benefits. That’s it, there’s no minimum income requirement or provision that you must purchase a home in the United States. If this were to become law, Canadians who meet the re-requirements would immediately be eligible to spend up to eight consecutive months in the United States, which is two months longer than the current limit of six consecutive months."
They are also lobbying the provincial governments to extend the medical, out of province coverage to coincide with the 240 days.
More here.
The CSA - Voice of Travelling Canadians - Canadian Snowbird Association Type Visit USA Act in their search option.
And as for taxation concerns, we will be filing IRS Tax form 8840 - Closer Connection Exemption Statement for Aliens. The form, in essence, acknowledges that you met or exceeded the "substantial presence test" BUT are not going to be filing a U.S. income tax return due to the fact that you maintain "a closer connection" to a foreign country, such as Canada, where you will be paying annual income tax.
This info is also available on the Snowbird website.
You are considered a US resident if you meet the substantial residence test for 2012. You meet this test if you are physically present in the United States for at least:
31 days during 2012 and
183 days during the period 2012,2011, and 2010, counting all the days of physical presence in 2012 but only1/3 the days of presence in 2011 and only 1/6 the number of days in 2012.”
Any day you are physically present in the US is calculated , regardless of how little time you spent there.