Israeli Prime Minister Netanyahu's campaign to have President Obama draw a nuclear "red line" that Iran cannot cross, amounts to having Obama issue an ultimatum that the US will invade Iran under the conditions specified in the "red line." From his UN speech it appears that Netanyahu sets his red line at Iran's production of an amount of uranium enriched to 20% that will produce enough highly enriched uranium to build a bomb warhead, according to Reuters. According to Wikipedia, the critical mass of U-235 metal needed for a warhead would be about 52 kilograms. Arms Control Now says 225 kg of uranium hexafluoride enriched to 20% would be about the amount needed to produce the uranium needed for a 90% highly enriched warhead. Uranium hexafluoride is the gas that is run through the enrichment centrifuges. Once weapons-grade enrichment level is reached, the uranium metal is separated from the fluorine to produce the warhead. Wikipedia says weapons grade enrichment is usually more than 85% U-235. (Natural uranium contains only about 0.7% of the U-235 isotope.) However, Arms Control Now reports that according to the last two IAEA reports, Iran's stockpile of 20% enriched uranium went down because Iran is using it to fuel its research reactor which needs 20% enriched uranium fuel to operate. The latest IAEA report for August 2012 said that Iran had only 91.4 kg of 20% enriched uranium, considerably less than the 225 kg it would need for a warhead, according to IPS News. The IAEA report for the previous quarter said that Iran then had a 20% stockpile of 101 kg.
According to IPS, the IAEA report also said the total amount of 20% uranium hexafluoride produced by Iran rose from 143 kg in May 2012 to 189.4 kg in August 2012, or about 15 kg per month. At this rate, to go from 91.4 kg to about 225 kg would take about 8 or 9 months, if no enriched uranium was diverted to the research reactor or other uses. In that case, Iran would reach Netanyahu's red line around June of 2013.
Arms Control Now says that once Iran has 225 kg of 20% enriched uranium hexafluoride, it would take about one more month to increase the enrichment to 90%. It would also take time to convert the uranium hexafluoride to uranium metal and then to fabricate the warhead.