http://www.ibm.com/developerworks/linux/library/l-job-terminating/index.html

Say you need to debug a pesky problem by running some traces for 30 minutes at midnight, or you would just like to use your Linux system as an alarm clock. This tip helps you stop jobs, such as those started with the cron and at capabilities, after the jobs have run for a certain time, or when some other criteria are met.