It's hard to quantify how often a diversion occurs, because by definition, it's not something one can predict...it can happen any time. During private flying, a few times perhaps. When flying fire, it was a regular thing...very frequently I got sent to a different airport while I was either returning to a fire, over the fire, or on the way back. Doing fractional work, it happened a few times a month...ATC would call us and tell us that the company wanted us somewhere else. I've had customers on board request a diversion a few times. Several diversions for weather, maintenance, etc. One for a passenger in left seat of a light piston twin one night, who had a heart attack. Most recently we diverted to Bahrain out of Kabul, and back to Europe, instead of going to a different intended location, and a few weeks ago we were diverted to Ostend instead of Brussels, and then sent to Brussels anyway when the weather cleared.