1) Once the visa is stamped on your passport you must complete your record of landing in Canada within 1 year of the date you took your medicals. This means there is usually less than 1 year from when you actually get the visa stamped.
2) Once you have landed in Canada, you must stay in Canada for 2 years (730 days) out of every 5 years in order to renew your permanent residency. You may, however, apply for Citizenship once you have been in Canada for 3 years (1095 days) out of any 4 year period as a resident.