If it were me, I'd be tempted to use a simple checkbox + edit field along the lines of
 Split into subtasks of [x] minutes
where the value of x is editable up to the duration of the event. This could be under the "Time Period" section or possibly in its own "Subtasks" section
On OK this would create the required number of consecutive events in the specified time scale. The default name of each event would be the description with Slot 1, 2, 3, 4 appended, eg Haircut (Slot 1). This could then be changed afterwards if necessary.
You would of course need to validate the value of x and show how many repeats were going to be created. You would also need to report/handle if the overall event duration isnt an exact multiple of the subtask.
You should also allow specifying a number of subtasks (eg 4) rather than a duration of subtasks (eg 45 mins)
If this was a rare use case you may want to consider hiding it under a button so as not to confuse the majority of your users.