I watched a really good Day 9 episode recently, giving lower-level players a sense of when you can go in for the kill. In short, you count the number of major advantages ("blunders") you get over the other player (losing a battle, you destroy an expansion, you successfully harass and do a ton of damage), and if the answer is more than two or three, you try to go in for the kill (build mostly attack units, mass up and go for the big finishing attack). Day9 suggests that number of major advantages really depends on your own play and your ranking, and you're going to have to figure it out how many advantages you need before you can go finish your opponent off, but this "blunder counter" gives you a method of deciding when to push.
Generally, immediate counterattacks after winning a single major battle can lose you the game, because by the time you get there, you hit both his fixed defenses and his reinforcements, while yours are further and further behind you. This is a "wait for another blunder" moment, or a "get more ahead" moment, rather than a "go frickin' kill him" moment. All of these moments are explained by Day9 in his various 'casts.