They last forever IMO
I put an itsy bitsy of my painterly pp in sample jars (the black screw top MAC gives out) so I don't need to open the real packaging too often. Painterly is very matte and so can easily dry out (my friend's has). It still works very well until today..
the reason that paint pot samples are dried out it's probably in the store itself, it gets exposed a bit too much that you can't help it from drying out..