Near as can recall, the standard approach when belt ratting is to chain the rat: killing the battleships, but leaving the frigates and the cruisers, so that the battleships will respawn. Some folk will kill off the entire spawn if the battleships have low bounties. See the old Provi Ratting Guide
for more details. This approach guarantees a certain level of income, since the destroyed battleships will always respawn, although not necessarily in the same belt, since the spawns will bounce from belt to belt, and from belt to gate at random.
In theory, if you're the only person belt ratting in a system, you can kill off all the spawns in the hopes of getting faction or an officer spawn, which can be worth hundreds of millions in items dropped. However, most alliances and corporations require you to chain rats when in a populated system.