How often and when does a molly mule come into season?

According to this site:

http://horses.about.com/od/understandinghorses/a/m...

"Despite being considered sterile, mare mules and mare hinnies will have estrus cycles. These cycles can be regular, or erratic and variable."

So basically, it depends on the animal.

I know with horses it can be quite varied, I've seen mares who come into heat every month (I owned one, unfortunately... haha), and others who only seem to come into heat a couple times a year.