All babies are different.
Every baby will learn at his or her own pace. I have a friend who's son was standing unassisted at 4 months old, but I also have a friend who's daughter hasn't made it there and she's pushing on 6 months. I think it's important to make sure your baby is learning out to use all of his or her parts, tummy time is a good way to do that. I bought a tummy time pillow and it has worked wonders. Tummy time helps your little one strengthen back and neck muscles that will help oil the entire baby machine :)
Does Charlie enjoy tummy time?