I love this question. I hope someone who knows what they're talking about answers it.
I've never written a series. As a reader, I feel that less recap is more. I often pick up number 2 of a series at the library, by mistake or because number 1 isn't available. I don't care much what happened in number 1--it's what is happening to the characters in number 2 that keeps me reading. I prefer the backstory tucked in quickly and sparingly at the points where the information is crucial to understanding the story, not dropped in as a prologue. In the event I have read number 1 first, I "skip recap" in number 2 and hop over anything I know. If I have read number 2 first and later go back and read number 1, I think, oh, cool, that's why that happened in number 2. I still enjoy both books, but probably not in the exact way the writer had intended.