I think, I have fund the answer for my previous question:
How long is a permanent resident visa valid?
A permanent resident visa is issued for a period not exceeding the earliest expiry date of the following documents:
the medical results for you and your family members or
your or your family members' passport.
Important information. Permanent resident visas cannot be extended once issued.
If applicants do not use the visas within their validity period, they must re-apply for immigration to Canada.
Their sponsor will have to submit a new sponsorship application and pay new processing fees.
It is NOT VERY LIKELY, that they would issue a VISA with a short 2-3 weeks expiry date
VERY LIKELY they will wait until MEDICALS expired and than ask for a re-do of them
(or if they are nice, they would extend MEDICALS for 30-40 days)