My second son pooped every 10 days. No joke. Breast milk is so completely absorbed, that not all babies even make enough waste product to poop, then he would have kind of a blow out, but basically, it was just every 10 days. I took him in a couple times out of fear and in the end, there was nothing wrong, and not it isn't overly common, it isn't unusual nor bad that t hey aren't pooping. My first however was a poop every feeding kind of kid. The second kid was MUCH nicer as far as diapers are concerned. I am hoping and praying my baby that is on its way will be more like the second.
Oh and my daughter who is two, when she was getting BM only, she was an every 4 day pooper, but since she didn't live with me (surrogacy created baby) She did get formula sometimes, and when she had formula she pooped a lot more. I guess my babies just like the bm better.