Legally, you must be in Canada* for 730 days out of the 5 years immediately after you become a PR. i.e. starting counting from the day you land. If you enter Canada before that 5 year anniversary, you need to have enough days left before the 5 year period finishes such that the number of days you already have behind you in Canada PLUS the number of days that remain before the end of your first 5 years is at least 730. i.e. You don't have to already have 730 days, but you do need to show that you will be able to meet the requirement by the time the 5-year anniversary is reached.
After the first 5 years is finished, you need to have been in Canada for 730 days out of the previous 5 years at the time your residency obligation is being assessed (typically when you enter Canada, apply for PR renewal, or a travel document).
*Some excemptions count as being in Canada for maintaining PR status, e.g. living with a Canadian citizen spouse outside Canada.