As far as ammo goes, how long it last depends on how it is stored. Protected from the elements, it can last for years. But...there's not only the issue of expenditure, but having the right ammo available for the weapons you have.
If a zombie plague went on long enough, requiring long term survival amidst it, probably for awhile, fortress towns would form, with walls, guards and strict control of entry. If they were lucky, they could produce their own ammo, and would probably restrict the type produced, to maximize humbers.
There is something else to consider, ammo is heavy Rick or anybody else could only carry so much the weight would make you as slow as a zombie. A city block of apartment in Atlanta would have somewhere around the hundreds of units with thousands of potential zombies. I know America is supposed to be gun crazy but every shot has to be a head shot. Those bullets which are heavy to carry won't last long as soon as you leave your fortress for food, water, looking for your mate....you are one firefight away from being unarmed.
Temis the Vorta wrote:
Seems like it would take less effort to whittle a stick to make a reasonable crossbow arrow vs producing a bullet (at home? I have visions of Colonial housewives pouring metal into little round molds by the fireplace but I guess it's more complicated nowadays?)
For a M16, you would probably blow your hand off if some random surviving housewife or Deputy Sheriff tried I imagine the same poor outcome. So far I haven't seen any hardcore survivalist, shooters, hunters.. on Walking Dead