Basically, when you feel ready to do it and stick with it. If you're finding it easy to squeeze meals in between milk feeds and naps, etc. then no harm starting one now. If you're still trying to figure it all out, there's no harm in waiting either. We started with one meal a day at 6 months, but didn't really add in a second until 7 months. She wasn't eating much before then, just playing, so I didn't see the point of stressing about adding the next one. But by 7.5 months, we were doing 3 meals a day.