Hi,
Most of us pay a huge amount in promotional fees, so I've been turning mine on and off at various intervals during the day.
I'm not exactly sure what duration to have, but I tried turning my adds on and off every 45 mins or so. While sales are different each day, on the days I tried it, it didn't seem to have a negative effect on orders, but there was a definite reduction in the amount of sales that were sold "via promoted listings".
I believe that when the promoted listings are paused, the listing continues its current position in the search results for a while, before sliding back down into the oblivion.
Two rules would probably be needed. One for the daytime, with a standard "on" and "off" schedule, but then an option to leave them on during the twilight hours.
This is also ideal for sellers like myself, who sell made to order products. At busy times like Christmas, when sales are higher than output, using this feature could reduce fees and control sales.
Ideally, each individual campaign would have these settings available.
Maybe others can do some testing - or just vote for it and give it a try if it goes live.