Throwing items is a standard action, obviously. That means by process of elimination, preparing an item in need of preparation to be thrown is an MEA. The number of weapons which can be thrown in a round, therefore, is determined by the individual's BAB: A full attack would allow him to hurl one item per +5 BAB.
However, if the item is not thrown on immediately following preparation, something bad could potentially happen. In the case of a grenade, for instance, pulling the pin, and then not throwing it at somebody, generally results in having it blow up in your hands. Thus, it's an MEA to pull the pin, a free action to shout "FIRE IN THE HOLE!", and a standard action to throw.