Having flown a variety of domestic flights in premium economy, I will say that I am unsure if distance, time of day, or serendipity is the driving factor for the hot sandwich option.

At least half the time I've received the sandwich on the CGH-SDU route, and that's blocked at 35-40 minutes or so in the air. Mind you, that used to be viewed as a more "premium" route, so that could be part of the equation, but regardless, you might get a sandwich on this short flight.

But I do agree with the above statement that the longer flights normally do come with a free goopy cheese sandwich. I can't speak to the longer longer flights to the North and if they get anything else.