How often do I have to renew my Polish permanent residence card, and does the status itself expire?

It is important to separate the status from the card. Permanent residence (pobyt staly) is granted for an unlimited period, so the right to live in Poland does not lapse simply because time passes. What does expire is the plastic residence card that proves your status.

You must apply to replace the card before it expires, and also if your details change or the card is lost, damaged or stolen. This is an administrative renewal of the document, not a fresh assessment of whether you still qualify, so it is far simpler than the original application as long as you do not let it lapse.