I see nobody's addressed your question, so I'll give it a whirl:
How long before the air is used up? Well, it's long enough for the airborne agents to disperse to the point of safe levels. That's all that is needed.
Nobody is claiming plastic and duct tape will keep you alive for a week. You only need minutes to hours to hang out in there.
In fact, this brings me to an observation on fighting these types of weapons. Except for vaccinations for bio, all measures are just stop-gaps, in order to give the victim time to move out of the area, or time for the agents to disperse into the atmosphere. Iodine pills to protect your thyroid gland in the event of a nuclear attack just keeps the strontium 90 out of there by blocking it. Eventually, you'll die from the iodine if you don't get out of exposure. Those insta-hypos the GIs have for nerve gas only slow the effect of the agents on the central nervous system. That will keep them alive just long enough to evac to the medics, where they get the real treatment.
Now, all that said, it's all for naught if the terrorists decide to insert their agents through a building's ventilation system.