Since I frequently need to leave for a flight before the checkout time, I probably only ask about 20-25%of the time, and it's usually only for an extra hour or so. I've almost always been successful. I do remember being turned down by the Park Hyatt (when it existed) in BA (and I was a diamond at the time) and somewhere (I think it was an R-C) in the US. I also remember that the Cairo Marriott OFFERED let me stay until 6 pm without charge because I had a late flight.