You can delay for as long your visa is valid. Once the visa expires, you cannot be granted entry into Canada.
Usually the visa is valid for a year- the year begins from the time CIC receives your medical results.
Good Luck on getting your Visa .
Medicals are Valid for 1 year so Embassy ll grant visa before you medical expire (Saw a person got just 4 days before expiry date)
My medical was done on 9 july 2015. According to embassy the visa process after medical was for 6 months . So we send an email and got reply from embassy that they ll take 6 more months for process and do not send any query in between that time period.
Then i got visa in feb 2016 with date of expire 9 July 2016
As members told you its valid until your medical expire. If unfortunately you did not take flight you visa ll expire and you have to go through medical process again which will be real pain.
My best advice will be to fly before expiry date.