How many years is the average for "normal use" after which it's expected to become difficult to read?
People who purchased Nano S devices in 2017 are starting to see first issues now, that means cheap display is probably meant to last 4 years more or less.
Ledger warranty is two years and they are not responsible for your device after that, but like I said you can fix this ordering new display for $2 or $3, don't waste any money purchasing new ledger.
However, there could be an issue with their new
closed source firmware breaking devices, like this guy reported few days ago (post is being downvoted on purpose):
I have been in crypto for a while and trusted Ledger. I bought a Ledger Nano S back in 2017 and after a compulsory firmware update the screen stopped working. The characters in the screen are not visible UNLESS I turn off all the lights and get to see some very light bright on the screen. I share this problem with Ledger support team and the only advise is to restore the wallet but no other solution as the ledger was not under warranty anymore.
One of my friends have the same problem, this is an issue with the ledger and has nothing to do with the users. You cant trust a wallet which after some time and due to internal issues basically stopped working. More than anything, their support team wont provide any solution at all.
https://www.reddit.com/r/ledgerwallet/comments/qqsyov/ledger_nano_s_not_working/