Iran has enough centrifuges and enough low-enriched uranium that it could, if it threw out the UN inspectors, probably make a nuclear weapon in a matter of months. Perhaps, if things went badly, it would take a year. But that would be a pretty primitive weapon, heavy and big, hard to deliver. It certainly wouldn't fit on a warhead of a missile, and it would be only one.
It's hard to come up with a rationale for why somebody would develop just one primitive warhead, because if everybody knows that you have nasty intentions, you have only the one missile, the one warhead, and it's now exploded. So the general feeling is that Iran would do well--maybe they won't take this advice--to wait until it has more low-enriched uranium and more centrifuges.
Furthermore, it's also quite possible that what Iran might decide to do is take all this technology that it's been developing and develop a second covert system, smaller presumably, where it would develop the material for one bomb, which it could then have without the world community knowing about it. Then, once it exploded that one bomb, it would have this big pile of material we know about, this low-enriched uranium, that it could quickly convert into a number of additional bombs.
However, there are real technical barriers that Iran has yet to overcome to figure out how to get to a warhead on a missile. They seem to be working on that. The one issue on which the IAEA has had the least success in getting any Iranian responses has been about the documentation for a project that Iran was working on about how to fit a warhead into a missile. Iran claims the documents are fake, but refuses to answer a whole lot of IAEA questions about that project.