How often will quads over quads happen?

[davidross]

I've played about 500,000 hands in the last year, and maybe close to a million hands online since I started playing poker, and I haven't seen quads over quads yet. In fact I've never seen a hand that would qualify for a bad beat jackpot where Aces full loses.

I've seen straight flushes lose (1 card flush though), and I've had 6 or 7 Royal flushes in that time, but I've yet to see quads lose. I wonder if the expectation is more often than once in a million hands.