Tic-tac-toe is a broken game. Once you figure out the few tricks, there is absolutely no way anyone can make you lose. My wife and I always end up with stalemates. The only one who loses is my poor niece.
I don't think you can actually make an equation for skill, time spent and game knowledge to decide level of play though. There's a lot more to take into account, like risk taking, creativity, adaptability etc.
Sometimes, you get those guys who have near perfect flowcharts, amazing reflexes and joystick control, but you find one simple loophole in their game and they fall apart. Then there are the guys who are not that great at anything but just somehow guess right all the time.