From the CIC website it looks like visit visas are limited to 6 months per visit.
Multiple entry visa
While valid, a multiple entry visa will let you travel to Canada for six months at a time as many times as you want. It will be valid for up to 10 years or one month before your passport expires, whichever is shorter. You must arrive in Canada on or before the expiry date on your visa.
Technically they could make a trip out of Canada and come back. When I had a residence visa in the UAE and I was studying in India I had a similar limitation that I could not be out of the UAE for more than 180 days ( roughly 6 months ). So I used to take a trip to the UAE and stay for a day and fly back the next day.
Looks like OP did not get spousal open work permit which would have let them stay in Canada with OP without this restriction.
Source:
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=417&top=16