When you log in, you will see a box that says Keep me logged in on this device. Here is what happens when you tick it, and when you don't.
If you tick the box
If you leave the box empty
Either way, you will need to log in again if:
Keep your account safe
While you are logged in, anyone using your device can see your information and make changes, just as if they were you.
On a shared or public device: