Expirity date of immigrant visas coincides with the day you underwent medical exams. For example, if you underwent medicals on Dec 01 2014, your immigrant visa will be valid until Dec 01 2015. If your spouse underwent medicals after you, she/he will have the same expiration date as you.
Processing time for PR Cards is 26 days after landing plus an additional 2 weeks for mail delivery.
It is not recommended to leave Canada before getting your PR card, unless you want to use your immigrant visa in that way. However, if you still have your student visa valid after becoming PR and before getting PR card, you can leave Canada and show your student visa to the airline when coming back. Here in Canada you can show your CoPR to enter the country. This is at your own risk, because CoPR will clearly say NOT VALID FOR TRAVEL.