Research has shown that once oxygen runs out in a sealed room, death is not instantaneous as was shown since the body shuts down in stages. For example, a 10x10x10 room would have about 17 hours of air for one person, yet they would not die until about 30 hours had passed, although severe effects would begin after 24 hours. Using the formula provided, Michelle in a 6x8x7 room would have about 6 hours of air. She would be endangered after about 8 hours and would die in about 10 hours.
Sam estimated Michelle would have 8 hours of oxygen based upon her estimate of the room being "six by eight by seven feet". Using a standard online calculation, she would actually only have about 6 hours.
The van was observed on the warehouse footage but did not go onto the freeway. This would have drastically limited the search area. Sam opined the vehicle could average 75 MPH, which is simply not possible on city streets. Starting the search from the last-known location would have resulted in Michelle being located in time.
Sam tells Michelle to keep talking. While the difference is minimal, the best way to minimize oxygen use is to sleep, which is what Sam should have encouraged her to do.