The duration of the first and second forward, as well as the duration of the neutral frames can vary in order to still perform the iWR2, however, pressing 2 exactly one frame after the third forward is mandatory.
For this example we'll be looking at Dragunov's instant while running 2 (iWR2), But this method works for all moves with the input (fnfnff+2).
While running moves can be done at range 0 consistently and without that much effort. You don't need to mash forward so fast your life depends on it. The only thing that matters for iWR2 at range 0 is that you press 2 exactly 1 frame after the third f press. If you press them at the same frame, you will get f2. If you have greater than 1 frame delay, you will get f2.
You can buffer the first f, in an iWR move during the recovery of another move, such as a WR 2 on block. This can mitigate some of the execution.
The trick here is to buffer the first forward, and then on the first frame when you can act, press: f, n, (exactly one frame of f), f+2.
This makes it quite a bit easier, given that you have a very large window to buffer the first f. All you need to focus on is timing of the second f upon the first actionable frame, and pressing 2 exactly 1 frame after the third f. A common way this is messed up is when the second f is pressed before the first actionable frame. When this happens you get ff2.
In this frame-by-frame overlay, the purple frame denotes the first frame of actionable input.
Take note of how early the first f is buffered.