What happens when the time runs out

When the Sharing Duration you picked runs out, the session ends on its own. Your Position (what other apps call your location) stops being sent, your friends stop seeing a live dot for you, and the capsule at the top of the map disappears. Nothing asks you to confirm it and there is nothing left to switch off.

It ends only that one session. Any other Group you are sharing with carries on for its own length of time.

Nothing renews itself. If you want to keep sharing, open the Group and pick a length of time again — and it is worth doing that before the countdown gets to zero, because tapping a different length while a session is still running moves the end time without a break. See Start Sharing with a Group.