Probably about 30 minutes in between. I had time after Spectro last month to find another spot to view Wishes, go change the baby's diaper, and then had probably another 10 minutes after then to wait.
I would think most people would stay, but we're planning on doing the same as you on our trip in May. We'll be in MK on Monday and viewing Wishes then. The next day, we're going to AK for 1/2 day and then finishing up MK that afternoon. We'll stick around for Spectro, but as I'm sure the kids will be exhausted, we're not planning to stay again for Wishes. I'm going to try to find a spot for Spectro on the right side of the street just before the bridge going into Tomorrowland - you know Spectro runs backwards from the Share a Dream Come True parade, right? - then I can hopefully make an easy exit after it's over.