I have never seen these before. But I think the time estimates would depend more on the ambient temperature. If you are laying on the with your back then the times would be shorter unless wrapped in a towel, which would absorb the water that would leak while they melt. That is the other concern is water leaking out and maybe making a mess depending on the surrounds.
But it seems like it might have a place in your toy box if it could work for you. Also depends on the price. You could just put the keys in a jar with water and freeze it.
