For many this may be quite an obvious thing, but it went unnoticed in our app for a year so I still think it is worth sharing.
We had over 70.000 entries in System.ScheduledEventInformation.
As it is a system table, I asked support what to do about it. The answer was quite simple: there is no problem in deleting old data from it.
Even if you did not create any scheduled events, the table might still fill up because other modules use them. Like CommunityCommons and the iCalendar module. This may go unnoticed because the System page that displays them no longer exists in many templates.
Just for the fun of it I created a model share, it uses a constant for the retention value.
https://modelshare.mendix.com/models/e933f85b-8a26-46c5-b178-602a96a22464/scheduled-event-cleanup