The RFC for iCal supports up to a recurrence at the second level. While I think that is too much, you should be supporting recurrence at the minute level. The Windows Task Scheduler supports this, as well as Linux/Unix Cron. If you are using the Scheduler Widget as a UI for a job/task scheduling system, then having the ability to schedule at the minute level is important. This is related to the feedback request to "Add hourly recurrence editor to Scheduler".