In fact, since the Japanese didn't know about radiation poisoning, neither city was uninhabited for even a single day. And for months afterward there was little knowledge of an special danger in returning to the cities. The U.S. imposed a lot of secrecy about the bombings (pictures confiscated, news reports suppressed) and the official information was misleading.

The official report of the Manhattan Project investigating the bombing of Hiroshima wildly underestimated the medical problems resulting from radiation. (The official report of the Manhattan project is available on Kindle for free. It makes pretty interesting reading. Plus it always shocks other Kindle readers to find that while they are reading Barbara Taylor Bradford, you are reading about the bombing of Hiroshima.)

How much danger was there from radiation for those who remained in the cities? Or traveled to them in the days after the bombings? Don't know. Not sure it can be known.