The 5/10 years estimate deserves an explanation.

The 10 years seems to be the general consensus that floats around most experts and intelligence agencies, assuming Iran maintains its program on a steady clip and doesn't hit major technology snags. From what I understand, it's based on what is known of the current state of Iran's nuclear program, reasonable assumptions on resources and know-how, and comparisons with other military nuclear programs in the past. Iranians seem to be serious, competent and well-organized and there is no reason to believe they won't get there if they are decided to get there. But it's going to take time and they are not there yet.

The 5 years lower bound is not really an estimate but more of a standard cover-your-ass disclaimer, based on the 3 years it took to the Manhattan project from its founding to building weapons:
- On one hand, most of the science and technology that the Manhattan project had to invent is now in the public domain.
- On the other hand, the amount of resources and talents the US threw in this effort was absolutely staggering, mind-blowing, earth-shattering (throw in any superlative you want, it deserves it). There's nothing in human history that compares to that, save, may be the race to the Moon.

A third-world country has a strong head start on the Oppenheimer team and doesn't need to reinvent the science behind the bomb. But it cannot replicate the resources. So counting the amount of time to build the nuclear piles, the facilities to reprocess the fuel and extract plutonium, do the research for the weapons, and get the whole thing running, it all comes to about 5 years.