Once a year for me, especially on young horses. Though in my old age I've become kind of a coward and have developed a habit of "bailing". I really don't like to get hurt, so I just jump off and do ground control until the problem is over. But I always get bucked off at the most random moments. Just horses being horses I guess.
Just once, and I wasn't even completely on yet. I was in mid mount about to sit and put my right foot in the stirrup when he shot forward, bucked and reared. I probably would have stayed on if I hadn't gotten the horn to the ribs. That horse was always trying to get me off!
Haven't been bucked off in a long time. Hunter tried a couple of times when I first started riding him but he doesn't much now.
The only time I remember being bucked off was when I was in my teens and worked at a riding stable (sure I fell off lots lol) In those days I wasn't afraid to ride nothing except one horse. Baldy - only horse I was afraid of until my 40's and I got Hunter. Unfortunately my guide horse had to be used for a trail horse and there was only one horse left in the barn... Baldy. So I had a trail ride of about 12-15 people and half way up the field he exploded, holy crap I thought I could fly.
I try and avoid bucking and rearing too. Hunter wetn through that stage and almost went over with the trainer on him.