Vladimir Putin's warnings against military action against Iran deserve to be taken very seriously. Since we're not contemplating actually conquering Iran and trying to occupy its territory, people need to understand that the post-strike diplomatic environment is going to be much more important to the future of the Iranian nuclear program than is any damage that bombing Iran with our on-the-table options might or might not do. If Russia decides to just send some scientists with schematics and materiel over to Iran and show them how to build a nuclear bomb, then -- bam -- nuclear bomb.
Conversely, at the moment not only is Iran under some diplomatic pressure to stop short of weaponizing, many countries around the world are taking direct measures to prevent the Iranians from just easily going and buying the stuff they need. Insofar as an unprovoked American military attack convinces other countries that the real dangerous lunatics live in DC rather than Teheran, countries around the world could cut back on their vigilance and make it much easier for an Iranian nuclear program to succeed.
The point is that when people talking about the Iranians being such-and-such time period away, or some bombing effort taking them back x number of years, they're talking as if progress toward a nuclear weapon proceeds at a constant pace. In practice, one of the factors that determines how quickly you can proceed is the international context. Right now, things are pretty tricky for Iranian nuclear scientists. Military action that doesn't reflect a firm, UN-backed consensus grounded in some reasonable interpretation of international law (military action that does reflect such a consensus seems very, very unlikely but in principle it could happen) could dramatically alter that.