The PR visa is usually valid for a year from the time you did your medicals. If your visa is so delayed that the medicals are about to expire, they will usually ask you to repeat them but it is possible and has happened to people that they get the PR visa with only days to land in Canada. Usually you have at least a few months.
You do not have to move to Canada immediately when you land. You can land and then go back to your home country to tie up loose ends. You only need to meet the residency requirements of living in Canada for a total of 2 years in each 5 year period.
