As this Reuters piece points out, it depends who you talk to.

There are estimates ranging from two months to two years, depending on what kind of bomb we're talking about and whether you believe the Iranians are further along in enrichment technology than we think.

There is little doubt that if Iran kicked inspectors out of the country that they could probably enrich their stockpile of LEU - Low Enriched Uranium - to the 85-90% threshold necessary to construct a bomb in a matter of months. The problem is in figuring out how far along they are in building a technologically feasible delivery system like a missile:

A paper published by the U.S. Bipartisan Policy Center think-tank said Iran could make 20 kg of HEU -- a quantity it said would be enough for one device -- in two months.

It said it remained unclear if Iran had mastered the technology to turn the HEU into a weapon, but that history suggested this could be achieved in less than six months.

But another Washington-based think-tank, the Institute for Science and International Security (ISIS), said Jones's calculation method was "unreliable" and a breakout in such a short time at Iran's Natanz enrichment site was not realistic.

Other experts stressed that Iran would also need to turn any weapons-usable uranium into the core of a nuclear missile if it wanted more than a crude device, adding to the timetable.

Mark Fitzpatrick, of the International Institute for Strategic Studies (IISS), said he now believed Iran could make a nuclear weapon in less than two years' time.

"Suggestions that Iran will be able to produce weapons in a matter of months are irresponsible," Fitzpatrick, a director of the IISS Non-proliferation and Disarmament Programme, said.

But, "just as exaggeration is irresponsible, so too is complacency," he added.

Creating enough HEU to build a bomb is only the first step. Do they have a workable bomb design? This missle question is critical because a warhead design is a very tricky proposition - especially for a third world country like Iran with limited technological abilities. Then there's the question of their progress with enrichment. They are using a centrifuge method to separate the isotopes which demands extremely thin engineering tolerances to make the cascade work. Even without the Stuxnet worm, they were suffering from regular breakdowns and failures. These problems didn't stop the enrichment - but it would slow down any program where they wished to enrich to weapons grade capacity.

The Israelis think the Iranians are two years away from having a workable weapon. Iran "is moving ahead in all of the ways that you would need to if you wanted a nuclear weapon," Fitzpatrick said.

Raising the pressure, U.N. nuclear watchdog chief Yukiya Amano this month said he was "increasingly concerned" about possible work in Iran to develop a nuclear missile. He hoped to give more details soon about the basis for those concerns.

Israel and the United States, Tehran's arch foes, have not ruled out military action if diplomacy fails to resolve the row. Israel bombed an Iraqi reactor in 1981 and launched a similar sortie against Syria in 2007.

"Israel has no doubt that Iran is pursuing nuclear weapons," the head of the Jewish state's atomic energy commission, Shaul Chorev, told member states of the U.N. nuclear agency last week.

Israel's chief of military intelligence, Aviv Kochavi, said in January that Iran could produce bombs within two years.

I would add that all this speculating and calculating depends on the notion that we know about all of the nuclear facilities that Iran possesses and that there are no hidden enrichment plants or other projects that would narrow the time frame for Iran's construction of the bomb.

Iran can't enrich their stockpile to the 90% bomb grade level unless the IAEA inspectors at Nantanz are kicked out. That's where the nuclear material is stored and the inspectors can keep a close eye on it as long as they are allowed to do their jobs. (Again, assuming there is no other site where enrichment is done in secret.) Given that, Iran would most likely put itself in a position where all the elements for making a bomb or two would be in place so that if they decided to go ahead and build it, they could bring everything together in a matter of months.

What we can be sure of is that very soon, they will have the capability to build a bomb but probably won't construct one. The dilemma for Israel and the US is to decide when that point is reached and whether to act or not.