I think for BF'ing to be anything close to resembling contraception you need to be feeding 4 hourly around the clock.
The agreed things you need to do to rely on BFing as contraception (about 98% effective when these conditions are met) is called the Bellagio Consensus, and requires:
  1. baby under six months old
  2. baby must be exclusively feeding from the breast (no pumping, even)
  3. feeding should be on demand and at least this often: every 4 hours during the day and every 6 at night
  4. your period must not have come back yet (not counting post-birth bleeding, obviously)


This info is from Lactational amenorrhea method - Wikipedia, the free encyclopedia

So it really depends on your feeding frequency, I guess. Of course, even if you meet all those criteria there's still a small chance, just as there would be with other contraception. (The 2% chance though is not 2% for every time you have sex, it's that 2% of women will be pregnant after a year of using this method... not that you can even use it for a year, but you get the idea.)