Durations with Quick Add aren't very precise. The best I've been able to find is to include "for {duration}" with the entry, where duration is for hours or minutes. So, in your case, something like:
Flight BAW182 at 10:55PM EDT for 8 hours
Unfortunately, while I can get it to create entries with a different duration than my default, it's really very basic. For instance, all of these failed:
Flight BAW182 at 10:55PM EDT for 8.5 hours
Flight BAW182 at 10:55PM EDT for 8 hours 40 minutes
Flight BAW182 at 10:55PM EDT for 8:40
Flight BAW182 at 10:55PM EDT for 8:40 hours
I don't see a way to add an end time, either. All of these failed:
Flight BAW182 at 10:55PM EDT until 10:07AM BST
Flight BAW182 at 10:55PM EDT to 10:07AM BST
Flight BAW182 at 10:55PM EDT end 10:07AM BST
Quick Add really seems to be for very basic calendar adds. Anything that gets outside of "something at some time on some day" just doesn't work that well.
(The information in Google Calendar support isn't particularly helpful, and certainly not in this regard.)