Exact Answer: 1 Month to 10 Years
Many Hollywood movies have envisioned their point of view about the apocalypse. The reason for the apocalypse can be an alien invasion, zombies, meteor or asteroid disaster, nuclear accident, natural catastrophe, world war 3, or any other reason. Whatever may be the reason, in an apocalypse, the protection of an individual & their family is highly dependent on arms & ammunition.
Depending upon the reason for the apocalypse, the shortage of arms & ammunition may occur. If it is a zombie attack, alien invasion, or a world war, then there are high chances that the ammunition may end sooner. If the apocalypse occurs due to a viral outbreak, asteroid crash, or natural catastrophe, then there is a higher chance that the ammunition may not exhaust soon.

How Long Would Ammo Last In An Apocalypse?
The shelf life of any modern ammunition is at least a decade. Even if we assume that all the ammunition manufacturing industries & units are shut down, existing & manufactured ammunition will be available for at least a decade.
The most severe problem would exist when there is the constant use of ammunition. This happens when you need to constantly fight with the attackers, like zombies, aliens, or maybe humans. In such a condition, constant production of ammunition would be necessary. If the production of ammunition is shut down, then considering the available ammunition on the shelf, hardly one may have a month or two.
Even if the production units of ammunition were active, it doesn’t ensure availability. Transportation & logistics for ammunition always remains an issue. The ammunition may last for a year or two, but getting hold of ammunition is a much bigger task.
Ammunition encompasses a broad spectrum. If the people are capable of designing & manufacturing their ammunition, then the ammunition would last more than a year. An extreme shortage of special caliber bullets and other such ammunition will occur. The usual, mass-produced ammunition will last longer.

Type of Apocalypse | Time Duration of Ammunition Available |
Zombie | 1 Month to 1 Year |
Alien | 1 Month to 5 Years |
Virus | 5 Years to 10 Years |
War | 5 Years to 10 Years |
Natural Catastrophe | 1 Month to 10 Years |
Meteor Crash | 1 Year to 10 Years |
Nuclear Accident | 1 Year to 10 Years |
Why Would Ammo Last So Long In An Apocalypse?
If the ammunition production units are working, and the logistics & transportation are no longer an issue, then it creates an unlimited supply of ammunition. Apocalypse is not a utopian period. Even if somehow the production units are running, the logistics can be a huge problem. Imagine zombies or aliens on the street. In such cases, the stocked ammunition is the only source.
Countries like the US, where the citizens are allowed to have arms & ammunition will have large stocks of ammunition. Many people know how to manufacture ammunition from easily available resources. This knowledge will play a crucial role as a deciding factor. Easily available ammunition types that are available in large quantities will last for years, while the special types may get exhausted or hoarded.

The ammunition available is dependent on the type of apocalypse. If there is a need for constant firing, like in a zombie or an alien invasion, then the supplies may exhaust more rapidly. In an apocalypse, the human population dwindles quite rapidly. This leaves a large chunk of ammunition available for the remaining population. The only problem is to get hold of this ammunition.
Conclusion
In an apocalypse, ammunition is a resource that will be scarce. In an apocalypse, arms & ammunition are necessary for protection. In an ideal scenario, the ammunition may never exhaust. It is because the production units will constantly manufacture ammunition. Apocalypse is not an ideal situation. Considering the worst & unimaginable situation, ammunition in the stock can last up to a decade.
Constant use of ammunition will deplete the scarce resource. If the people have the skills of manufacturing ammunition with easily available resources, then the ammunition will be available in plenty. The availability of ammunition is directly or indirectly related to the cause of the apocalypse.
The article leaves a lot of room for interpretation and speculation. The variables seem quite uncertain in real-life scenarios.
I believe that the author is analyzing the worst-case scenarios, which is a good approach to understand the risks.
I completely agree with your comments. It is hard to predict what would truly happen.
The author provides a good perspective on the relation between human population decline and availability of ammunition.
The article is very informative and provides a very well-detailed insight on the amount of time ammunition would be available in different types of apocalyptic scenarios.
I agree that it is very well detailed.
It is intriguing how the author relates the availability of ammunition to the cause of the apocalypse. It’s something not often addressed.
I also found that part very interesting. It provides a new way of looking at things.
The article leaves room for a lot of speculation, particularly regarding the production of ammunition in extreme situations. Very thought-provoking.
The author’s discussion on the worst-case scenarios does make you think about the potential risks and outcomes, which is commendable.
The possibility of humanity dwindling in the event of an apocalypse is interesting. It really makes you wonder how much ammunition would really be available.
It’s intriguing how the article addresses the skill of manufacturing ammunition with easily available resources. A well-researched piece.
Absolutely, the strategies for securing ammunition are quite fascinating.
This article is highly informative and thought-provoking about a topic that is often overlooked. It definitely leaves a lot to think about.
The approach to analyzing the worst-case scenarios is very logical and provides valuable insights.
I agree, understanding the risks is important.