0.24 seconds is a maximum figure since valve closes for about half the cycle only !
a milestone to pass, yes it is !
I never did valves so small, and neither did measures on the valve's sealing...
I simply put a piece of tubing in the cylinder port with its valve (without spring) and with a good breathing in with the mouth by the other end of the tube, the tip of the tongue feels easily the depression, which must be hold for more than about 5 sec, and its done.