If you are not breastfeeding "ecologically" (I believe that's the term used to distinguish from the widely misused and misunderstood "exclusively; also called natural mothering) you can get pregnant/period at anytime. Even after the first few weeks after delivery.
Ecological breastfeeding means that not only are you breastfeeding exlusively, but you are breastfeeding constantly and on demand such as in cultures where the child is worn at the breast at all times so the baby is constantly feeding for both nurishment, and just for comfort instead of using a pacifier.
Ecological feeding also includes keeping your child at your breast while sleeping so the baby is comfort suckling and feeding on demand throughout the night. Bottom line, in order to effectively space children using breastfeeding, the child has to be feeding constantly, on demand, during the day, and most especially at night.
Many people misunderstand how using breastfeeding to space births works because in western cultures we believe that if we don't introduce formulas or bottle feed (even using breastmilk) we are effectively spacing our children. Where this method fails many women is when we feed on a schedule. Schedule feeding(even a fairly rigorous one) doesn't trigger the appropriate hormone levels necessary to suspend menstruation. Only feeding on deman and constantly can effectively ensure a total suspension of menstruation. As a result many people are often surprised to find they are pregnant a month after delivery even though they are breastfeeding and only breastfeeding. So to answer your question, even with a somewhat irregular cycle, you probably stand a high chance to get pregnant/ovulate within the next 2 to 4 weeks of beginning to wean, and subsequently see your AF return shortly thereafter if pregnancy doesn't occur.
While challenging, spacing using breastfeeding is doable and can be very effective, allowing you to hold off pregnancy for up to a year or more if done "ecologically". I'd recommend you consult a La Leche League consultant or the LLL website www.lalecheleague.org for more information on how to do this effectively. Also read "Breastfeeding and Child Spacing" by Sheila Kipley. It is the best explaination of how to do this and in a simple easy to understand way.