Ok,
so I would use a timer then. There are two Nodes you could use to achieve that. The “Set Timer by Function” and the “Set Timer by Event” Nodes. These Nodes call a Function or Event after a given amount of time and can be reseted. More here
You should create a variable that holds the Timerhandle you recieve from the "Set Timer Node. When the AI Actor looses sight, you call a “Set Timer” node. And set the Timerhandle variable to that returned Timerhandle. When the AI Actor regains sight you check if a timer exists and then delete it, if it does using the “Clear Timer” or “Delete Timer” Node (sorry don’t know the name of that node) and clear the Timerhandle variable.
The Function or the Event which is called after the time reaches zero than handles the rotation backwards.