Depending on whom you talk to, the Iran deal's duration is either its greatest weakness (only a few years and then we're right back where we started!) or one of its greatest strengths (resolving the nuclear issue for half a generation!). In either telling, you will hear people shorthand the deal as lasting "10 years" or "15 years" or sometimes even more.
How is it possible that the deal's duration could be both a strength and a weakness, and that people could give totally different numbers for it?
This all makes a lot more sense if you look at this chart, put together by the pros at the Arms Control Association, clearly showing how long the various provisions of the deal will last:
The first thing you will see is that the deal does not "end" after 10 years, or even after 15 years. That said, it is true that there will be more restrictions during those first 10 and 15 years than after. The provisions that end after 10 years include the number of centrifuges Iran is allowed to have and the restriction barring it from researching more advanced centrifuges. The most important provision that lasts for 15 years is probably the cap on the amount of low-enriched (energy-grade) uranium that Iran is allowed to have.
So what this means is that after 10 and 15 years pass, Iran will be allowed to have a larger nuclear program, but one that is still under constant monitoring and inspections. Those latter 10 years, from year 15 to year 25 (roughly the period from 2030 to 2040), are years when the deal will still be in force, and when we will still have a very good sense of whether Iran is holding to its obligations, as well as a strong ability to detect if Iran tries to cheat and to make those detections quickly enough that we could act before it got a nuclear bomb. That is how the deal works in the first 10 years and how it works after that, so it is just not remotely accurate to say that the deal ends in 10 years or even 15.
That goes to show why it's so crucial that inspections are what last the longest, running for 25 years at Iran's only two uranium mines, meaning we will know exactly how much uranium (nuclear fuel) the Iranians have taken out of the ground, which means we'll know if they're siphoning some off to a secret nuclear weapons program. That provision doesn't end until about 2040. Iranian Supreme Leader Ali Khamenei would be over 100 by then; Iranian Revolutionary Guard Commander Qassem Suleimani would be about 85. There will be a new generation of Iranian leaders in power.
It is just not the case that as of year 11 or 16, or even 21 or 26, suddenly the deal vanishes and Iran is allowed to build a nuclear bomb. Iran's commitments under the nonproliferation treaty bars the country indefinitely from building a nuclear weapon. Iran has also agreed to implement something called the Additional Protocol, which comes with pretty tough restrictions and also continues in perpetuity.
So the next time someone says the Iran deal only last 10 years or 15 years, show them this chart.