According to former international weapons inspector, David Albright, Iran has time to hide the evidence before the inspection of their formerly secret nuclear site.
From the LA Times:
Allowing access within two weeks of the announcement would in effect give Tehran almost a month after its Sept. 21 acknowledgment of the plant’s existence to obscure evidence, they said.
David Albright, a former international weapons inspector and president of the Washington-based Institute for Science and International Security, said it would probably take Iran some time to conceal activities. But, “if you have a month, you have the time,” he said.