The whole procedure is extremely involved and technical.
Gas masks have to be used and only well-trained technicians are employed.
One should not reenter an ordinary building within that period of time, because there is little if any forced ventilation. Furthermore, ordinary items like carpets, drapes, furniture, and so on lengthen the time required to restore fresh air. Also, there is a tremendous safety factor allowed for. Safety standards don't apply in wartime, and especially not when the aim is to kill a thousand people as quickly as possible. The Germans had plenty of experience with gas in general, and Zyklon in particular, since it was used so often in delousing.