It doesn't really matter how much longer your PR card is valid for. What matters is whether your PR status is still valid. Someone can hold a PR card that is valid and yet because they have been outside of Canada for most of the past 5 years and will not be able to fulfill their residency requirement in the time left in the 5 years, they can be found inadmissible for failure to meet their obligation.
If this is the first 5 year period since you became a PR, you will not be turned away. You have 15 months or approximately 450 days out of the 730 required to maintain your status, and another 2 years in which to meet the remaining 280 days.
If you don't want to keep the status after this period, I suggest you find out from CIC or CBSA what you need to do in order to voluntarily relinquish it. Do not just let it "lapse". The danger of just letting it lapse is that although you are visa exempt and can enter the country freely as a tourist, you may still be reported as a PR who has failed to meet his residency requirement and find yourself ordered to leave the country by being found inadmissible for this failure! Since you don't want to keep it, let it be known. Don't just assume they are going to relieve you of your residency obligation because you made a decision not to remain a PR. Advise the proper authorities and deal with it the same way you dealt with your application to become one: according to the proper procedure.