Comfort nursing is totally normal, and they do it pretty much until mama puts her paw down about it. Now, that can vary wildly. I've seen mamas put an end to nursing around three months, and more than one who continued until the kitten was a cat. One for a good two years. LOL, some of us remember one member whose kitten began nursing his Lab, who began relactating ( continuous nursing can trigger milk production in ALL mammals of either sex), and was doing so for over a year! I wouldn't be surprised if the little rascal was still nursing from that dog!They still try to eat from mama though, am I not feeding enough or do they just like mama more?
There is no way for you to "over feed" kittens this age. Their little metabolisms are going ninety to nothing, and they are growing like weeds. Feed them as often as and as much as they want for now.