I would say about 4 (my baby is 10 months), but it really depends upon what you mean by 100% active paying attention. He's awake for about 10 hours a day for reference.
I pay 100% attention during all times that I am caring for him (feeding, changing, etc), and I try to play with him for a hour or two a day or so, but in between it's definitely "spot" attention.
He's my third, so he's definitely entertained by all the activity going on with the others, the dog, the cat, etc. He's also often a "tag along" for other things. So, he's in the car a lot for drop offs and pick ups, in the stroller for walks (with the dog), in the stroller for my exercise, etc. Having him in the stroller, to me, does not mean I am paying him constant attention.
Trust me, there are days, especially like yesterday when he didn't take an afternoon nap, where he's attached to me and I am holding him (or else he's crying), but even then I still have stuff to do, so I'm holding him AND doing something else -- so I'm still not paying 100% attention to him.
The rest of the day, he's wandering around the house (he's not huge into sitting and playing with toys either) from room to room, person to person, checking things out and otherwise entertaining himself.