Lawful permanent residency does not expire. Green cards (which are used as proof of LPR status) expire, but you can keep renewing the card without applying for citizenship.

https://www.uscis.gov/green-card/after-we-grant-your-green-c...

https://www.alllaw.com/articles/nolo/us-immigration/failed-r...

However, if you do not apply for citizenship once you are eligible, you lose certain legal protections against employment discrimination:

https://www.justice.gov/crt/8-usc-1324b-unfair-immigration-r...