Permanent residency is permanent. You can only lose it if you commit a serious crime or if you fail to live in Canada for at least 2 years in every 5 year period or if you are found to have lied on your PR application.
There is a PR card you sometimes need if you have to prove your PR status and also need to show in order to return to Canada when you have travelled and that card has an expiry date of 5 years. When it is about to expire, you simply apply for a new one. You can renew your card indefinitely as long as you meet the residency requirements. You do not have to become a Canadian citizen.
