From the opinions I've read this is almost exclusively a sign of waning interest/disinterest.

Well, I met a girl in A different city a couple of weeks ago who I've been exchanging messages with - met through a friend and had a long night which culminated in us being back in my hotel room. We both take pretty long to respond to messages, yet all she can talk about is fucking me again and she has just booked a flight to come here pretty much exclusively to bang - she's paying, not me. What's the logic here?

By long I'm talking usually 30 mins or so, but sometimes it can be a couple of hours depending on the time of day.