Yeah, the speed of the rock/bb in the cap will vary a lot depending on how far it depresses the valve stem core, and the tightness of the thread on the cap. Minutes seems absolutely unrealistic though; at the rate of the video you shared that's probably a good 6-8 hours. I'm sure with some optimization (larger "rock" to press the valve in further and a maybe some cuts in the the thread to allow faster release of air), you could get that down to the hour or two range, but you'd also risk someone finding it easily as the noise of escaping air would be quite a bit louder.