You can verify that the timeout is in effect by running a query on the SYS_SESSIONS table (to list the JDBC sessions), and looking at the TIMEOUT_DURATION column in the results. The TIMEOUT_DURATION column should contain the value that you set for the Default Session Timeout, in milliseconds.
Below is an example for a Default Session Timeout of 18 seconds.
SELECT * FROM /services/databases/system/SYS_SESSIONS ss WHERE ss.SESSION_TYPE = 'JDBC'
Result: STATUS : TIMED_OUT TIMEOUT_DURATION : 18000 ACTIVE_REQUESTS : 0