all the information and answers you ever want ref visitors or immigration can generally be found on the CIC site including answer to this http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=1016&top=16 As said above if passport not stamped that generally implies 6 months as the CBSA officer is trusting to a degree the visitor will honour that , if less or a specific purpose will be stamped with an actual date.
