It only took ten or so years for Hiroshima and Nagasaki to become habitable again, but those were very small bombs compared to what we have today. That said, I am not sure I would do anything. I mean, if a nuclear missile is coming for Houston, that likely means the entire country is getting peppered, meaning nuclear holocaust. Given that we would then launch our nukes in response, basically vaporizing most of the world, leaving the rest to deal with a nuclear winter...I'm not sure I wouldn't rather die instantly in million degree heat, rather than waiting to die slowly from radiation poisoning.