How often does the AngularJS digest loop run?

Angular digests are triggered – they don’t happen through polling.

Code executes, after code is done, angular triggers a digest.

Example:

 element.on('click', function() {
     $scope.$apply(function() { 
         // do some code here, after this, $digest cycle will be triggered
     });
 });

Angular will also trigger a $digest after the compile/link phase:

Compile > Link > Digest

And as to how many digest cycles are triggered? It depends on how soon the scope variables stabalise. Usually it takes at least 2 cycles to determine that.