There is probably an exact timing, but trying it to get a feel for it in practice runs might be better. Watch runs to try to see how early people can do it and not miss the skip, how early they did it and missed the skip (check VODs at the relevant times).

For example when Flowey closes the game, I heard it was about a second after the screen goes completely white is when you can open the other client (Flowey will autoclose the main one). I tried it in practice a few times, did it in a run at what I thought was the same time, and loaded into judgement hall. Close game, restart, and it worked as it should.

Or the 6th soul phase, I've heard it's when you see the first green healing item, or when the music changes, but I've also seen runs where TGH reset because he didn't hit that trigger even though there was a smidgen of green on screen.

There isn't really "this exact frame is correct" info available for it, at least not that I know of.

If it's taking that long to become playable, are you sure you're playing the game outside of Steam?