I'm sure there are people far better qualified than me to answer this but as they haven't bitten yet, I'll give you the benefit of my dubious wisdom. The answer is 'not very often', which might be a bit vague but I present it to you with the best intentions.
We do see the AN124 fairly frequently, there was one here at the weekend. Which doesn't help you I'm afraid. I think there was an AN12 in a couple of weeks ago as I caught a glimpse of it as I drove past.
Ugly buggers, aren't they?