A well conditioned (and well cultivated) student should stumble back at least 15 feet. It makes your demonstrations more impressive.
Seriously though, even in push hands a training partner shouldn't move that much if he is practicing his skill rather than just helping you show yours. That's a low-level demonstration of kung fu, or showmanship if you like.
With real fa jing the opponent should (ideally) drop. More realistically, the body reacts to a fa jing strike as it will to any good boxer's punch.
"Look, I'm only doing me job. I have to show you how to defend yourself against fresh fruit."
For it breeds great perfection, if the practise be harder then the use. Sir Francis Bacon
the world has a surplus of self centered sh1twh0res, so anyone who extends compassion to a stranger with sincerity is alright in my book. also people who fondle road kill. those guys is ok too. GunnedDownAtrocity