I was just wondering the same about my 3mo son. He seems to have a really long attention span, and be perfectly happy playing by himself. I try to check in with him regularly, just to remind him I'm here and see if he's ready for something different. He just grins at me and babbles, then goes back to what he was doing. I almost feel like I'm interrupting!
I've been trying to balance it with plenty of 1-on-1 time, but... sometimes he just really wants to be left alone with his mobile, or in his swing with his pooh bear, or beating up the toys on his bouncy seat toybar, or chewing on his hands and watching the ceiling fan.