A little reminder of how we don't really know anyone's story except our own. I personally have heard each of these. Have you?
You're so lucky that you get to stay home with your kids. Maybe she is thrilled to be a stay-at-home mom. Maybe she wishes like anything she could have kept her career. Maybe it was a no-brainer decision for her family. Maybe it was a difficult, perhaps agonizing decision for both partners to make. Maybe finances are ridiculously tight and the family is struggling. Maybe she does consider herself lucky. Or maybe she feels trapped. You have no idea, so don't assume. On a related note, please no I don't know how you leave your children every day to go to work comments either, okay?
I support breastfeeding, but [cover up, don't do it in public, you should wean by this arbitrary date, etc.]. You either support breastfeeding mothers or you don't. Once you start putting qualifiers on it, you're being wishy-washy, and wishy-washy is not support. And it probably goes without saying, but for the love of squirrels no negative comments to a mom who formula feeds, either. You have no idea why another mother has chosen to feed her baby the way she has, and frankly, it's none of your business.
I don't understand why people say they need a break from their kids. I will speculate that mothers who say this have a supportive partner, probably one who works a job with regular hours and is home in time for dinner most nights and every weekend. Maybe they have parents or in-laws or close friends nearby. They are likely not in a situation where they are going it alone, either permanently (single parent, divorced, widowed) or temporarily (spouse is military or in a career with significant travel). Their "breaks" may just not be as obvious as a night out or a regular babysitter, but having someone around to share the daily mental energy matters.