CP Session expires after short time
99% of the time when you see this behavior, it's because wherever your server is configured to save PHP's session files to either doesn't exist or isn't writable by PHP.

You can check where that path is by going to yourdomain.com/admin/utils/phpinfo and searching for session.save_path.

If you have the overridePhpSessionLocation config setting set to true, then Craft will set that path to craft/storage/runtime/cache.