Is there a timeout for a query over database link?
Since you're using dbms_scheduler, you can set the max_run_duration attribute of the job to some limit, and then have the scheduler email you if that event is raised.  Behind the scenes Oracle uses queueing tables (which can enable you to create jobs that fire when an event is raised, if you wanted to take the extra steps to do more automation around your response).  But basically any job that runs over the max_run_duration you setup will raise event type: JOB_OVER_MAX_DUR

The email piece is built in 11gr2, see here for good writeup.

Hope that helps.