What should you play if you have very little time to study? One answer is to adopt the Colle System which acquired the name Businessman’s Opening as it requires very little time to learn and maintain. White plays 1.d4 followed by 2.Nf3 and 3.e3 almost regardless of how his opponent answers.
There are two different flavors of Businessman’s Opening, the Koltanowski treatment with c3, Nbd2 and Bd3 and the Zukertort treatment with Bd3, b3 and Bb2. Which is better? Well it depends what Black does. And against certain Black set-ups it might be better not to play the Colle System at all.
Here are a couple of Youtube videos which give an idea about how to play this opening. There are also other resources such as the DVD I did for Chessbase.