In theory the status is for ever. In practice the card is only valid for ten years.
In practice it is for ever as well. But yes the card is only valid for 10 years, automatically to be renewed. The interesting bit here is that is NOT a karta pobytu. Mine is a salmon-red piece of paper looking as a passport page printed on 4 sides, stating 'Dokument potwierdzający prawo stałego pobytu".
It is my understanding they are only issued to EU citizens, but I might be mistaken.
And you cannot get one of these if you do not have a "meldunek stały".