Some babies spit up often. And some babies throw up often. My son sure did. It lasted until he was about 3 months old and then just stopped.
First things first, know that babies absorb all the nutrients from breastmilk very quickly, so even if they don't keep the feed down, they are still getting all the goods.
Does it happen regardless of what you do with him? I mean, bouncy seat, hold him, lay on the floor, etc? Is he in pain? Is he eating too much?
It could be reflux, but I feel that babies with reflux do it more often. My son was doing it 2 or 3 times a day so 2-3 times a week I think is fine. If it escalates then I would worry more. And my son never went on meds and he got through it just fine. Please don't think I'm suggesting it's reflux, it's just the one thing I'm familiar with. It could also be something in your diet that is not agreeing with your lo.
Check out the reflux board just to make sure, but it sounds like everythign is healthy and fine. If you aren't sure, see your doctor. Mom's always know best.