When do I ovulate if I have irregular cycles?

Brittany
I have very irregular cycles lasting longer than 28 days. When would I ovulate? I'm assuming it is always different since ovulation happens so many days before menstruation begins.
Also, or in the alternative, what type of ovulation test are you ladies using if you are trying to avoid pregnancy? I'd like to use them to assist while using the natural family planning method of birth control. 
I'd love to hear your opinions or about your experiences with something like this!