I got mine back at 29 mos. While it was nice to have it absent, I was getting a little concerned when it didn't come back because I wanted to conceive again soon. At 28 mos I contacted Dr. Newman, wondering if it was the frequency of nursing, or the duration of each session that affected ovulation, and he said it's unusual for a mother to go even as long as I did without getting a period, but that even if the child was nursing 24 hrs a day, I would still get my period back eventually. And he was right, one month later!
He also said it was rare to ovulate before you get a period. It happens, but he said he wouldn't count on it or try to figure it out.
I got a lot of info and stats from the Couple to Couple League's book The Art of Natural Family Planning which gave a bell curve graph of when one got their period back. Depends on nursing pattern, of course, but also depends on person.
Not to worry!
PS- if you're sick of always using the diapragm, you might want to look into the Fertility Awareness Method which means looking at your temp, vaginal discharge and/or cervical position to help you determine when you're ovulating. Then you can have sex without protection on the days you're sure you're not ovulating. Pardon me in advance if you already know about this, or use it, or don't want to :-)