We have changed the cookie lifetime in Magento 2.3.1 to 29 days and also change PHP configs (session.gc_maxlifetime) to the exact same period.
We are using in Magento session saved in memcache.
Randomly and a few customers, I would say 1 in each 50 is having issues accessing the website after some time idle. (Meaning they accessed the website before, added some items to cart and left the window opened, few days later they come back and try to navigate, then get timeout.)
Any thoughts that what could it be?