15.47. The length of time the thwomp stays at the top and bottom of its cycle is random. The time at the bottom varies by about 7 frames, while time at the top varies by about 25 frames. The thwomp starts at the top when the level loads, so the RNG is relevant three times: when it is at the top at the start, when it's at the bottom after that, then whet its at the top for a second time (this in when you jump off it). This means there is a variation of about two seconds for exactly when the thwomp starts descending on the cycle you want to jump off it, and to get cycle you need pretty good RNG.