I am a U.S. citizen.  I own a home in Washington State, and would like
to purchase a 2nd home in Vancouver, Canada.  I expect to go back and
forth across the border once or twice each month, never spending more
than 3 or 4 weeks in Canada, followed by a week or two in the U.S.  I
will not be working in Canada.  From an immigration standpoint, I
would like to be considered a "visitor", which means I can't spend
more than 180 days in Canada.  But I can't find any clear answer
regarding whether or not the 180 days is CONSECUTIVE, or whether it is
CUMULATIVE during any 12-month period or calendar year.  Does the
180-day clock reset each time I leave and re-enter the country?  I
already understand the TAX implications of remaining in Canada more
than 180 cumulative days. I need to understand the IMMIGRATION issue
only.

Hi retiree,

Briefly, you can make several small trips which add up to only 182
days within one 12 month period. Getting your head around how this
works requires an understanding of both the tax and immigration rules.

In regards to the tax issue, you can stay in Canada for up to 182 days
in a tax year. This is calculated by adding up all the days or parts
of days you spent in Canada, whether in a block or broken up
throughout the year (cumulative not consecutive). I've copied and
pasted a few relevant sections at the following websites, but please
click on the links for full details.

Residency status
"You're a deemed resident of Canada for tax purposes if you:
 * stay in Canada for 183 days or more (the 183-day rule) in the tax year;
 * don't have residential ties with Canada; and
 * aren't considered a resident of another country under the terms of
a tax treaty between Canada and that country.
If you're considered a resident of a country with which Canada has a
tax treaty, you may be considered a deemed non-resident for taxation
purposes."
The 183-day rule
To calculate the number of days you stayed in Canada during the tax
year, include each day or part of a day that you stayed in Canada,
including:
 * the days you attended a Canadian university or college;
 * the days you worked in Canada; and
 * any days or weekends you spent on vacation in Canada."
http://www.cra-arc.gc.ca/tax/nonresidents/individuals/deemed-e.html

Determination of an Individual's Residence Status
"¶ 20. An individual who has not established sufficient residential
ties with Canada to be considered factually resident in Canada, but
who sojourns (that is, is temporarily present) in Canada for a total
of 183 days or more in any calendar year, is deemed to be resident in
Canada for the entire year, under paragraph 250(1)(a) of the Act."
On the other hand, if the same individual were to vacation in Canada,
then he or she would be "sojourning" in Canada and each day (or part
day) of that particular time period (the length of the vacation) would
be counted in determining the application of paragraph 250(1)(a) of
the Act."
¶ 22. In addition to individuals sojourning in Canada for a total of
183 days (or more) in any calendar year (see ¶s 20 and 21), subsection
250(1) of the Act ensures that any person (other than a factual
resident of Canada) who is included in any one of the categories
described below, is deemed to be a resident of Canada."
http://www.cra-arc.gc.ca/E/pub/tp/it221r3-consolid/it221r3-consolid-e.html

NR74  Determination of Residency Status (Entering Canada)
http://www.cra-arc.gc.ca/E/pbg/tf/nr74/README.html

Ok, now we can bring in the rules at the border. As a visitor, you are
allowed to stay up to 180 days for each single entry, but even though
you can technically make two 180-day (or four 90-day, etc) entries
into Canada in one year, the tax-rule (which is cumulative) will come
into play.  In other words, although visitor status is consecutive,
residency for tax purposes is cumulative, so ultimately, you can make
several small trips which add up to only 182 days within one 12 month
period.

Consular Services for the U.S. Mission in Canada
"A visa is not required for U.S. citizens for a stay up to 180 days."
http://www.amcits.com/enter_canada.asp 

U.S. Department of State
"Canadian immigration officials at ports of entry will issue persons
planning to stay longer than 180 days a visitor?s record."
http://travel.state.gov/foreignentryreqs.html

Additional Links of Interest

Information on BC: Border and Customs in British Columbia
Visas & Immigration
http://www.britishcolumbia.com/information/details.asp?id=4

TIPS FOR TRAVELERS TO CANADA
Current Requirements for Entry Into Canada
http://travel.state.gov/travel/tips/regional/regional_1170.html

I was glad to work on this for you. If you have any questions, please
post a clarification request and wait for me to respond before
closing/rating my answer.

Thank you,
hummer

I searched the CIC and CRA websites as well as Google using terms such
as: canada canadian visitor status u.s. citizen retire 180 183 days
entry residence, etc.