The definition of post farming is; posting more then 5 times in under 10 minuets without any real meaning or contribution to the thread. That being said, adding a 30 second will slow down spam (however not stop it) and will not prevent post farming. No Support, it will be very annoying.
No support. The post farming rule is there for a reason. A 30 second cool down is too long. I think it is fine how it is.
