I got my periods back at 8 weeks despite never having gone longer then 2 hours between breast feeds and never haven given a dummy
But as a midwife I have cared for many women who fell pregnant before a period, a large amount around the 3-6 months post baby. However I have also met a large number of women who had to decrease feeding/wean their feeding baby in order to get periods to return to fall pregnant.
I'm not sure where I heard/learnt this but I think the number if months feeding equals for the length of time for normal menstration to return after the cessation of feeding. Eg- feed for 3 months = 3 months until period and so on.
I would NEVER recommend using just breast feeding as a sole method of contraception!!