It's generally a function of how much action you put in. In late 90's early 2000's I was playing around a million hands per year on single line. I would do around 20 to 25 royals per year.
The multi-line games came out about 2000 and that changed everything up as far as number of royals per year.
On a royal promotion at Sands Regency Reno I hit 48 royals in 20 hours of play on a hundred play machine. It was interesting when I caught a 4-card royal on the flop (happens about every 2700 games). It only happened I think 4 times. You get 100 draws to a royal. You should average 2 royals per draw but I caught 7 royals on one of the draws.