If the cronjob runs every 5 minutes, try this configuration:
Generate Schedules Every 5
(enter here the cronjob execution time, in this case 5 minutes)
Schedule Ahead for 125
(based on cronjob execution time plus the maximum time one job needs. For example: sitemap generation takes 120 minutes, then enter 120 minutes + 5 = 125 minutes)
Missed if Not Run Within 180
(runtime of the longest process, for example: an import takes 120 minutes, then enter 120 minutes + 60 minutes – because sometimes there is a difference between mysql and server time)
History Cleanup Every 10
(minimum cronjob execution time = 5 * 2 = 10 minutes in this case)
Success History Lifetime 1440
(duration of cronjob storage, to proof if everything works fine. 1440 = 24 hours)
Failure History Lifetime 1440
(duration of cronjob storage, to proof if there is an error. 1440 = 24 hours)
And last but not least, install AOE-Scheduler for a visual inspection of your cronjobs.
http://www.magentocommerce.com/magento-connect/aoe-scheduler.html
