DD is exclusively B/F on demand and I think the breaks between feeds might be an important factor. DD slept for long periods at night from the first few weeks, so while we fed like crazy during the day we sometimes went 7-8hrs at night... she is now 3 months and I am pretty confident I am getting to be fertile again.
I think it is quite dependant on your bubs rhythm and what your body does naturally...
hope it all goes to plan for you