The estimate starts with the calculation how much enriched uranium is needed for a bomb.
The amount of uranium has to exceed the "critical mass" for which the rate at which the nuclear reaction "feeds on itself" exceeds the loss of the nuclear reaction by escaping neutrons. In principle, this situation may be achieved whenever the percentage of the rare uranium-235 isotope, which is the fissile (i.e. it can sustain chain reaction), exceeds about 6 percent (the remaining uranium, uranium-238, the dominant one on Earth, isn't too useful for the reactions). At 6% enrichment, the critical mass is infinite and drops from infinity as you increase the enrichment level.
For a 20% "low-enriched uranium", the critical mass is about 85 kilograms for one bomb. Iran arguably has more than 75 kilograms of the 20% low-enriched uranium at this point. Given the estimated rate at which Iran is producing the 20% uranium in its centrifuges, they were estimating how quickly they could get the required amount for one bomb.
This estimate is probably overly optimistic. Not only they probably have some additional enriched uranium unknown to the nuclear watchdog. Also, they are probably able to increase the enrichment above 20% which means that the critical mass would be much lower. If Iran isn't there yet, it's probably a matter of weeks for them to get to the critical point. The prediction for the coming weeks or months is a combination of the extrapolation of the known centrifuges that produce uranium at a known enrichment level; and a speculation about the speed with which they are capable of improvements of their centrifuge systems.
At the same moment, the relevant Western intelligence agencies claim that Iran hasn't made the official decision to really build a nuclear weapon in a particular timeframe yet. This interpretation of the Iranian decisions is unfortunately a falsifiable statement and the falsification could hurt.