can anyone inform me of the upper limit to the standby->shutdown timeout?

But that is the whole point it is only the transition from standby to shutdown that is affected. i.e. if you wake up the player from standby it will play until your battery runs flat. And if it shuts down after waiting in standby for 2 hours, you can just turn the ignition to ACC and back to off and you're back in standby from where you can wake up the player.

I don't know what the maximum time is but something like the maximum value of a signed 16-bit integer would be logical to me (32767 seconds). Has anyone ever bothered to try?