Hi mala,
Actually we have the same situation, my husband is about to send my application on the 2nd week of May and he told me to do the medical on May and I dont need to wait for the medical request. But my concern is medical has an expiration of 6 months so we decided to wait for medical request before doing it to be sure.
1) Medical results are valid for 1 year.
2) Best to check your region specific guides. Normally for sponsoring family, providing proof of medical is required to send with the entire application upfront.
EDIT: Forgot this.
@ mala: It doesn't matter if you get your medical done just before submitting your application. Yes, it will probably take the doctor awhile to get the results back, and then a bit longer for him to courier it all to the regional medical center. But it doesn't matter, because all YOU need to provide in your application is the little slip he'll give you to prove you did your medical. When the visa office finally gets your file (way after Mississauga does) and needs your medical, they will use that slip you sent them to request your medical results from the regional medical center.
So don't worry about the doctor getting the results "late" and sending them in later. He sends them to the right place, your visa office requests it from them, done. It actually usually delays things to put off doing your medical.