... killall -9 acroread OR Ctrl+Alt+Esc. It's instant.
On the other hand, File / Exit, when a large document is open, takes forever and drives the disk light nuts for maybe a minute - like most laptops, this one has a really slow hard disk. I don't know the exact amount of time but given that Linux performs extremely badly when the disk is being accessed, it's too slow for me to want to try to reproduce it again.
As far as I can tell, no data is lost this way (maybe config settings but I don't mind) so it brings into question the purpose of writing teardown code if it performs so badly.