The other week on one of my permissions I dug 74 targets in a 6 hour session (counted them when I got in). So on average that's 12 targets per hour. The percentage of those targets that were any good is a totally different matter, but just about all of them were non ferrous items that have to be dug. That was on what I class as my "busiest" permission.
In contrast on one of my other permissions I can sometimes go 15-20 minutes between signals, I dare say there's people who can go longer.
As dave said, it's a question with many answers but a very interesting one anyway!