The visa you are issued to first land in Canada is valid until the expiry of your Medicals. Medicals are valid for one year after the examination. Practically speaking, most people have about 6 months of validity left by the time they receive their PPR.
Once you land, you are given PR status.Your initial PR card is valid for 5 years and can easily be renewed as long as you spend at least 2 out of those 5 years in Canada. Your PR status is at risk if you do not meet this requirement, but it is still possible to renew it albeit more onerous and not guaranteed.