WASHINGTON — No sooner had President Trump declared that he was exiting the nuclear deal with Iran than Hassan Rouhani, the country’s president, reassured the world that he remained committed to making the accord work — even if Mr. Trump did not.

But he also left a vaguely worded, dangling threat: If Iran’s foreign minister, Mohammad Javad Zarif, failed to keep the deal going with Europe, Russia and China, the clerical government in Tehran might conclude that America’s departure from the agreement freed it to speed up its centrifuges and begin enriching uranium at levels well above what was agreed upon in the summer of 2015.

Mr. Rouhani said that he would not bend to “psychological warfare” from Mr. Trump and that Iran’s Atomic Energy Organization was ready to produce more nuclear fuel if necessary.

All of which raised the question: If Iran tried to ramp up production, how long would it take to get enough fuel to build a bomb?