How long does poly cotton last?

[midlifecrash]

I have a couple of pillow cases, bought when I was about 12 and passed on to me when I left home. For about 30 years I have only used them as bags for delicate items in the washing machine. They are very faded but I noticed today that the fabric is still strong. Label not legible but I remember they are poly cotton, it was such an advance on the horrible nylon sheets we had before that

Nylon sheets -now you’ve mentioned them I can still remember the feel of them on my legs!

And the static!

We've got poly cotton pillowcases that have been so well used they are now just 'poly'. All the cotton has washed away. Poly(ester) is virtually indestructible so not planet friendly.