How to increase a scheduled time using a new field and rules component scheduler
What you'll have to do to get this to work, consists basically of these steps:


Create a new rule using event "After updating a node of content type Event, in which you add a Rules Condition to check that the value of the duration field was changed (I suppose you know how to write such Rules Condition).
The Rules Actions to be performed in this new rule to be created, consist of these 2 Rules Actions (in the order specified):


Delete the scheduled Rules Component that got scheduled when the node (Event) was created. That way the node will not get deleted as originally planned. For way more details on that, refer to my answer to "How to delete a scheduled task using Rules Scheduler?". Make sure to understand that it is extremely important, for this to work correctly, that you usve a unique identifier of the scheduled rule ... using the node title for it is asking for trouble (what if 2 nodes have the same title???), which is why I always make the node's nid part of such identifier.
Schedule the same Rules Component you already have (to delete the Event later on) AGAIN, using your "duration field".



What this really means is that your implementing logic that reads something like so "Don't delete this node (Event) after one hour yet, instead give me X more time (as indicated in your duration field) from now on (= the moment the updated node (Event) is updated with that duration field update)".