According to the U.S. envoy to the IAEA, Iran is dangerously close to a nuclear bomb: “This ongoing enrichment activity . . . moves Iran closer to a dangerous and destabilizing possible breakout capacity,” he reportedly told the IAEA board.

So how close are we to an Iranian mushroom cloud?

Analysts and government officials routinely offer different time lines for an Iranian bomb—but they tend to put Iran’s breakout capacity a few years away. Iran is experiencing significant technological difficulties. And the political decision to go for the bomb might not have been made yet. It does not mean that Iran does not intend to build a nuclear arsenal over time—to the contrary. But testing a nuclear device comes with a price: Iran is a member of the Nuclear Non-Proliferation Treaty. Crossing the nuclear threshold for a rudimentary nuclear device that Iran may not yet be able to replicate or deliver will be costly and fall short of achieving the strategic goals Iran is pursuing through its nuclear program—namely, the survival of the Islamic Revolution and its rise as the regional hegemon.

Tactically, Iran may therefore prefer to wait until it has accumulated enough weapons-grade fissile material to build not one but dozens of bombs; until it can build a nuclear device that is small enough to fit into a missile warhead; and until it has perfected its ballistic-missile technology to the point where a long-range missile can accurately hit a distant target. That time line is still quite long—years, not months.

However, it is not the time line that matters for policymakers. For long before Iran has accumulated enough fissile material to build an arsenal and enough technological know-how to make it into deliverable warheads, Iran will have mastered the technology and cracked the scientific secrets needed to reach that goal. It is the difference between knowing how to ride a bicycle and owning one. The regime is closer to the former than the latter—the U.S. envoy’s statement is further evidence of that. But once the knowledge is there, it will be harder to halt the march to the real thing. Thus, this shorter time line matters more than the actual moment when Iran will break away from the NPT, build several warheads, mount them on missiles, and threaten its neighbors.

But even this time line is not the one that policymakers must rely on for their planning. Long before Iran has built its arsenal or acquired the necessary knowledge, it will have shielded dozens of clandestine installations from a possible military strike. Iran knows that military planners in Israel and the U.S. constantly update their contingency plans for a strike based on fresh intelligence. The more Iran spreads its program, the more it hides it behind an impenetrable shield of defenses and fortifications, the harder the job for those in the West tasked with devising a realistic plan of attack.

At some point, they will tell the U.S. president and the Israeli prime minister that a military strike to retard or destroy Iran’s nuclear program is no longer an option. From then onward, Iran’s run to nuclear capability is unhindered. The removal of a credible military threat from the arsenal of diplomatic tools available to the international community will considerably reduce its leverage on Iran’s regime. Whereas the nuclear clock may be still ticking slow enough to give us time, Iran’s efforts to make its program untouchable are less burdened by scientific challenges—that clock is ticking much faster. Tehran will get there long before it can threaten anyone with a deliverable nuclear weapon. Once that happens— in months, not years—the game is up.