I've heard it is normal for babies to go for days without a bowel movement?

A bowel movement with every feeding is common in the early weeks. However around 6 weeks of age some babies change their stooling pattern from very frequently to once a day, or once every few days, to even once every week or so. Your exclusively breastfed baby is not constipated as long as the stool is still loose (pudding like) and a mustardy yellow colour when he does have a bowel movement. The stools should also be substantial. (Remember: the longer it has been between stools, the bigger it will be.) Expected bowel movements are summarized in Information Sheet #457 How to Know your Baby is Getting Enough Milk.