What AB said.
The timeout period is 20 minutes. So if you don't check off the "Remember me" box, and you take 20 minutes or more with no system interactions, you will be logged out. If you don't want to check off the Remember me box, and there are some good reasons not to (you're on a public computer at the library, etc.) then try to remember to do something to signal the system you're still around, maybe a page refresh in a separate window, or do a PM preview, etc.
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