From an American Enterprise Institute study, via Front Lines:

Earlier this year, the American Enterprise Institute’s Critical Threats Project concluded that Iran could have the fuel for a single nuclear bomb by September. That is a frighteningly short time away but luckily, Iran would still have to construct the bomb and fit it onto a missile. The question is: Where is Israel’s red-line?

The think-tank determined that Iran could make enough 20% enriched uranium for a bomb by June. From there, it would take only “two and one half months” to make the fuel for a 15-kiloton weapon, roughly the power of the atomic bomb that destroyed Hiroshima.