First, that's not exactly how probability works. On the second turn, it can either wake up now or the next turn, so it has a 50% chance of waking up. On the third turn, it has a 100% chance of waking up.
Second, this is not true for every generation. In gen 1, it lasts 1 to 7 turns, and the Pokemon doesn't move on the turn it wakes up. In gen 2, it lasts 1 to 6 turns. In gens 3 to 4, it lasts 2 to 5 turns. In gen 5, the counter always resets when switching. In gen 3, it usually doesn't reset but has weird interactions with snore and sleep talk.
