Minecraft wiki entry for Phantom, associated text: Phantoms attempt to spawn every 1–2 minutes. They spawn only if it is night or a thunderstorm is happening, the player is above sea level (y=64) with sky access, and the local difficulty is greater than a randomly chosen value between 0.0 and 3.0. The chance of a successful spawn is x−72000⁄x, where x is the number of ticks since the player last entered a bed or died. This roughly comes to a 1⁄4 (25.0%) chance on day 4, a 2⁄5 (40.0%) chance on day 5, a 3⁄6 (1⁄2) (50.0%) chance on day 6, 4⁄7 (about 57.1%) chance on day 7, and so on.