Your platform-fill tell is okay at smaller stops, but at somewhere like Atlantic Ave or Jackson Heights the platform never empties, so it’s easy to misread.
My cutoff is three minutes with no announcement and doors open, unless I’m in an air-conditioned car with a seat and no pressing appointment. If doors are closed and we’re between stations, I don’t consider walking, for obvious reasons. If we’re at a station and the conductor has gone silent after an initial 'train with a mechanical problem ahead of us,' I start checking the MTA app and looking at the opposite platform. Two empty trains on the opposite track in a row usually means I’ll leave.
On the Astoria line, I always walk from Queensboro Plaza to the 7 if an N/W sits there more than five minutes and I see no movement north or south. I’ve won that bet far more than I’ve lost. On the 6 at 125th, though, the M15 is a decent fallback, but only if you’re not headed to the east side below 59th. After that, bus lane or not, you’re in crosstown traffic.
One caveat: don’t bail just because the person next to you does. I’ve watched half a car empty out at Union Square only for the train to leave ninety seconds later. Make your own call, and if you’re going to leave, leave quickly — hanging in the doorway or turning around on the platform is how you get caught.