Good Morning JamFam!
I looked on the CIC website and it states:
There are two ways you can have an immigration medical exam, depending on your immigration category
► Wait for instructions once you have submitted your application
► Undergo your exam before you submit your application (Only for certain Family Class applicants)
► Fees
▼ Validity
Your medical exam results are valid for 12 months only. If you do not come to Canada as a permanent resident within that time, you may need to have another exam.
When I read the below link it looks like it is up to the VO to ask for an extension and this can only be done before the 15 month mark (i.e. expires at 12 months but they have 3 more months past this date to actually request the extension). This extension is asked for (and usually given) when the application is nearing the end of processing and it will only be the medical that holds up the end of the file. If the meds are extended then they are good for another year.
http://www.cic.gc.ca/english/resources/tools/medic/assess/cert.asp
Hope this helps. The link has a lot of good information and shows how the VO and Medical Examiner determine what to do. Having said all this we all know that KGN does not necessarily work the way the should.