パスポートの有効期限、もしくは、健康診断をうけてから12ヶ月以内(どちらか早い方)のようです。
For how long is my Immigrant Visa valid?
The validity date is based upon the earlier of: your or your dependents’ passport validity date(s); or the medical validity date(s). Medical examination results are valid for 12 months after the initial medical examination. Immigrant visas cannot be extended once issued. If applicants do not use the visas within their validity, they must reapply for immigration to Canada.
http://www.cic.gc.ca/english/immigr/guide-ce.html#faqより