When Jacob started teething, I laid off the tummy time for a few days because I was afraid he'd smoosh his face into the ground and it would hurt. After teething symptoms subsided a little, I figured we'd better get back at it because he still seemed like was struggling to hold his head up for more than a few seconds at a time.
I don't know what happened on those few days off, but when he went down on his tummy recently, his head shot straight up, and he held it up for at least a minute. Every time he's been on his tummy since, his head has been up longer and longer. He turns his head and looks to both sides and smiles and laughs while he's on his tummy.
It's like he turned three months old and BOOM! He gained head control overnight!