Yesterday, the best answer is: -ceil(log2(2x)).  (Contributed by fire. Good Job!)
turtle solved the puzzle at 02:08:21.643; length: 15
kaz solved the puzzle at 02:14:07.961; length: 15
kaz solved the puzzle at 02:16:46.247; length: 15
fire solved the puzzle at 03:58:36.338; length: 15