How long does it take for someone to die in a gas chamber?

It depended on the gas chamber. There wasn't one single standard size ...

It would take over an hour to fill a gas chamber (1500-2000) people because the changing rooms did not have a large enough capacity to let everyone undress at the same time. Once the doors were closed and the gas was put in the first would die within a few minutes (the gas would paralyze the lungs) the furthest away from the source would take anything up to about 15-20 minutes to die. It would take about two hours to clear the bodies out at which time the chamber would be hosed down and the walls whitewashed, the wet floor adding to the illusion that the victims were entering a shower room.

The gas was more effective in the warm and in wet conditions, in the winter portable heaters would have to be placed in the gas chambers as the gas took too long to sublimate (thank you to answers.com for helping me find that Word).

Also in the smaller room (2000 capacity being the largest) they put less gas in to produce approximately the same time scale.