Presumably as many times as someone like Abaddon or Huron can summon it up to crush something.
Difference with Exalted is that as rare as they are they can be beaten, Primarchs for example.
But, the Avatar took 12 transcendent gods who man handled his greatest servents to take down once and it didn't get a birth boost so it'll be just as powerful when its summoned next as it was now.
To make matters whose as has been stated the Avatar hangs around its not like an exalted that will disperse after a battle it can presumably command wars, and generally have an impact in excess of what an Exalted can hope to match while being incalculably stronger, especially since its not just a fighting machine its a god of war, so its likely the best strategist, tactician and leader in the galaxy.
Since I used them already, take the Primarchs, big honking threat that abaddon really hates. Even summoning up 4 exalted is unlikely to be enough to deal with the 4 primarchs, the Avatar if its recent showing is any indicator would eat them for lunch and by doing so crush one of humanities last bastions with ease.
Ridcully is incredibly influential, no doubt about that, but this is the closest Chaos has to an I win button. I'd call it a silver bullet, but really its a ****ing sledge hammer.