In short, as Iran is free to interpret the agreement, 63 or even 78 days may pass, plus three potentially lengthy periods that Iran can stretch out: One of “explanations” before the clock starts, one to agree on necessary means and “resolve concerns,” and one for advisory-board selection near the end.
So from the moment the IAEA first tips its hand about what it wants to inspect, likely three or more months may pass. All along, the Joint Commission is required to act in “good faith,” and to make only “minimum necessary” requests limited to verification, not “interference.” Tehran could also cite these terms to challenge particular requests.
The description of this process is based on the English-language text of the nuclear agreement. The text lacks a provision that it is the entire agreement, so Iran may claim support in supposed side agreements or statements during negotiations.