How long the inlet is open is not that important, itīs the time between cycles that is important. And you just need to know how much time you have at your maximum rpm. At half the rpm you have twice the time, but itīs not likely that your engine can handle twice the amount of fuel just because you have the time to inject it.

Itīs 60000ms in a minute and that divided by the max rpm will be:

60000 / 8500 = 7.06ms between tdc and since itīs a 4-cycle you get twice the time. That equals 14.12ms @ 8500rpm

The reason that itīs not that important to inject while the intake is open seems logic when doing some more math, The intake valve will open and close nearly 71 times every sec @ 8500rpm. The fuel will never have the time to stop by the valvehead and "wait" for it to open.