It's definitely once an hour so may not necessarily be at the top of the hour. I ended up with an animated avatar the other week, which was strange because you can't seem to specify an animated avatar in gravatar (I've tried but without success so far).
You are lucky if it's only happened once for you, kayakamina. Mine has changed at least six times. Fortunately it's never changed to anything embarrassing. I was envious though when Bibblebub had a great animated one last week