All pilots flying in controlled airspace must have a category 4 medical certificate (and have passed the HAGAR exam). For those pilots that have no medical conditions, this can be obtained by completing a Category 4 Medical Declaration and sending it to the local Transport Canada office. One can also visit the local transport Canada office and apply there in person (an appointment may be required). It may take 40 business days to get it back.
If there is a history of cardiac disease, or any other medical reason to doubt fitness for flight, then a civil aviation medical examiner (CAME) needs to make the decision whether one is fit for flight, and sign off on the category 4 medical.
Specific medical requirements for the declaration are here, in the last column.
This Medical Declaration does not expire by itself, as long as what is on it is valid.
Here is a link to a list of all the offices that can provide these certificates.
A list of Civil Aviation Medical Examiners can be found here.