We utilized RS three times today and here is what I have learned about the "extra" wait times:
The CMs at the FP queue are determining the "extra" wait time length. They manually program the time to return right before they scan the RS on to your park ticket. Seems to be based off when they think the 1st person will actually exit the ride after factoring in how busy the FP queue is and possibly the stand by wait time, but I don't think that factors as much.
At Screamin the CMs programmed my RS to be valid 5 minutes after my husband entered the ride as the 1st party. Or as they explained it to me, "basically when he gets off, you'll be able to go on".
At Star Tours the CM looked at the FP return time currently being accepted and said "ok, I'm going to estimate this" and programmed the RS to be valid 30 minutes after the 1st party. Not the hour wait it was yesterday in the middle of the afternoon when it was busy.
And it was an extra hour wait for Soarin, again in the middle of the busy afternoon.
However, in all three instances the window to return and redeem the RS was only ONE HOUR.
So it appears not to be an exact science. No standard extra "one hour" wait. Which I guess is good news, and potentially frustrating as well. It will be very difficult to predict whether RS will be useful on any given day or time as it now seems to depend on what the CM manning the FP queue thinks. ??? Hopefully others will contribute their experiences as well so we can pool our various experiences to find the best ways to use RS to continue the magic of Disney with younger children.