Generally I´d recommend at least 45 mins in a case like yours (carry on only and no visa issue due to german passport) because it depends which station your train will leave from.
DUS has two separate train stations - one with S-Bahn/local trains only inside the terminal and one at the other end of the airport to be reached by the people mover (or a bus at certain daytimes).
Btw. be aware that the people mover/bus is not free but requires the purchase of a ticket.
If you tell us where you want to go/which train you´re planning to take/from which train station you´re leaving ...
Otherwise it´s not possible to give a qualified answer.