Background Scheduled Jobs - Runs more Hours then expected!

Hi,

I have a job scheduled for delivery documents to run once in 15 Minutes. But the scheduled job goes more than 2 hours to complete each time. In this case, how does the job schedule work? It will work once in 2 hours (only after completing the previously scheduled job) or it will get triggered as per the job schedule definition i.e. once in 15 minutes.

I appreciate your help in advance.

Thanks,

Kannan