I only see 3 or 4ish a month, but they are generally all during the day.
It all depends what / how you are flying. A palyer that does the standard base centred low/med altitude stuff will see them less often, and mostly of the "bail after drop" type.
If you are hunting strat raiders, you will encounter them much more often, mostly of the "bail when enemy shows up" kind.
Just now I had upped for a suspicios darbar, which I thought to be a potential city raider.
And what happens after tracking down the Tu-2's at in my Ta152H 17k after about 20 minutes of searching? - As soon as I get into his icon range, AseHold bails from all three...