But I noticed that I can see "Mystler" at the Ferry Dock, though he isn't online since 20 hours.
The server links you out of the world after you've been disconnected for ten minutes.
EDIT-ADD: Sorry, misremembered my own code. It checks every three minutes, which means you will be linked out of the world 0-to-3 minutes after you disconnect. (I should make that a little more predictable.)
Mystler registers as still being connected. If you have a browser window/tab open, that keeps the connection open. In my experience, when you close the window/tab, that closes the connection -- but I'm not positive this is true of all browsers.
(If you leave the window open but find yourself disconnected, it's probably because I updated the software and restarted the server. I've been doing that every couple of days. You should see a warning first.)