In the meantime I tried a lot and guess I found the solution.
GetExecuteOnce eo = new GetExecuteOnce(t.getRunID());
conn.sendRequestAndWait(eo)
DateTime nextExecution = eo.getExecutionDateTime();
This gives me the next execution time wether it's a Single scheduled execution or a recurring one.
Hope there's someone who can verify that this is a valid way I'm going.
Thank you.