You need to spend 1095 days (3 years) in Canada as a PR to be able to apply for citizenship. These 1095 days need to be within a 4 year period.
If you were already in Canada before getting your PR, you can count each day as a half day towards the citizenship requirement but only the last 4 years before you apply count so the earliest you can apply after getting PR is 2 years and that would be if you were already in Canada for 2 years before getting your PR and never left during the entire 4 years.
