From my experience it is always best to marinate meat for a decent amount of time, so the meat can "soak" up the marinade and be more flavourful.
However, I came across this recipe here: http://www.abc.net.au/tv/cookandchef/txt/s2264630.htm
In the directions, it says: Add pork and marinade for a minimum of 6 hours or overnight, but no longer than 12 hours.
I was wondering, why does it say "no longer than 12 hours"? Isn't it good to let meat marinate for as long as possible?