From the Terraria wiki page on the Eye of Cthulu
While playing, the player may get the message, "You feel an evil presence watching you...", which means that the Eye will awaken soon. After a few minutes the message "The Eye of Cthulhu has awoken!" will appear and the boss will promptly start to attack the player.
In my experience, you'll see the 'watching you' message about 3 times before the Eye of Cthulhu actually spawns. Just keep playing. He'll probably spawn in a couple days game-time.
