Hardly ever, now. Had the first one I've had since my teens and twenties just a couple of weeks ago. Hadn't had sex for at least a week before that. Didn't feel that special, either, but was tickled to know I could still have one.
I didn't used to have that many when I was younger, probably because I generally kept it fairly regularly drained, but one time I came in my sleep-intensely- which made a total of 4 orgasms for that day. So you just can't tell.
They more usually happened if I hadn't cum for several days.
Sometimes they felt amazing, sometimes just like peeing a bit.