They lose pressure if stored for too long. How much probably depends on your sensitivity to lost of pressure.
Lots of people play with dead balls so a new can that has some pressure lost would still few "new" to these people. IF you are used to new cans every time to hit then you will notice the difference.
I found a mixture of 1 year and 2 year old cans( from when I purchased them) and all were dead in that they did not play like new ones. Multiple tennis partners confirmed that these were different than the usual ones.
Dunplop Grand Prix, Wilson US Open, Penn Pro all suffered lost of liveliness.
I heard that they are beginning to sell wine in plastic bottles, mainly to restaurants. Reason is that plastic is lighter so they can ship more fluids. Reason restaurants are the first wave of customers is because
1: the restaurant patrons will not know that it came from a bottle (selling by the glass only)
2: plastic bottles are not completely air tight, while glass bottles are. I didn't know this. So, I infer that plastic tennis cans are also not completely air tight hence the very slow loss of pressure.