Well, the 903 S-Bahn S7 from the Terminal Station (not the Mainline station) is the one you need to catch. If everything works fine and you are not too much in the back of the bus, you need 20 minutes (without luggage) from the gate to this station.
Normally the flights into DUS are very much in time (today ORD arrived at 707), so I would not worry too much.