My Canadian citizen parents have a permanent residence in Canada
however would like to spend time with me in the US. They will
regularly commute back to Canada however is there a limit on the
maximum number of days in a calendar year they are allowed to live in
the US e.g. 6 months? Please provide some official links with your
answer. Thanks!

citizens can stay in the United States for up to 6 months without a
visa.  The following website contains the answer from Customs and
Border Protection.  Cheers.


http://help.customs.gov/cgi-bin/customs.cfg/php/enduser/std_adp.php?p_faqid=619&p_created=1056630853&p_sid=RYkTz9ei&p_accessibility=0&p_lva=&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9MjEmcF9wcm9kcz0wJnBfY2F0cz0wJnBfcHY9JnBfY3Y9JnBfc2VhcmNoX3R5cGU9YW5zd2Vycy5zZWFyY2hfbmwmcF9wYWdlPTEmcF9zZWFyY2hfdGV4dD1sZW5ndGggb2Ygc3RheQ**&p_li=&p_topview=1