If the server is restarted, I think it invalidates all the active sessions.
Also if you have an HA setup and your Load Balancer in front of the web server is not set up for stickiness, you will need to relog every time the web server targeted is changed.
I do not know how to check for "time left". I'll reach out to a few colleagues to see if I can get an answer. Support may also provide an answer faster.