Preferences are completely personal and each piece of meat varies, so it's quite impossible to say that you'll get the best finish at N hours.
What acs says is true, the only sure way is to open it up and taste it. But of course, this means that you'd need to repack it and return it to the bath if it's not done to your liking.
It's also worth giving a pinch test to help you learn how to judge for future cooks. As you pinch it, try to remember the feel and spring that it gives you. Also observe the way the muscle fibres are reacting to the pressure. If they're opening up and being pushed away from each other, you've got your meat well into 'pullable' stage.