In 1000 cubic feet. 10x10x10 one person can live 17 hours. The last few will be nasty but you won't die until 17 hours.

2 people will last 8 or 9 hours. 3 people will last 5 hours. If you are excited, and breathing hard, cut those figures in half. If zombies are pounding on your truck, you will be excited.

T = Number of hours of air in a sealed room
(Volume of air inside the room in cubic feet) x (3% or 0.03)
T = ---------------------------------------------------------
(Number of people) x (one person's hourly production of CO2)

Concentration of CO2 in caving

0.03% Nothing happens as this is the normal carbon dioxide concentration in air.
0.5% Lung ventilation increases by 5 percent. This is the maximum safe working level recommended for an 8 hour working day in industry (Australian Standard).
1.0% Symptoms may begin to occur, such as feeling hot and clammy, lack of attention to details, fatigue, anxiety, clumsiness and loss of energy, which is commonly first noticed as a weakness in the knees (jelly legs).
2.0% Lung ventilation increases by 50 percent, headache after several hours exposure.

Accumulation of carbon dioxide in the body after prolonged breathing of air containing around 2% or greater will disturb body function by causing the tissue fluids to become too acidic. This will result in loss of energy and feeling run-down even after leaving the cave. It may take the person up to several days in a good environment for the body metabolism to return to normal.
3.0% Lung ventilation increases by 100 percent, panting after exertion, Symptoms may include:- headaches, dizziness and possible vision disturbance such as speckled stars.
5 - 10% Violent panting and fatigue to the point of exhaustion merely from respiration & severe headache. Prolonged exposure at 5% could result in irreversible effects to health. Prolonged exposure at > 6% could result in unconsciousness and death.
10 - 15% Intolerable panting, severe headaches and rapid exhaustion. Exposure for a few minutes will result in unconsciousness and suffocation without warning.
25% to 30% Extremely high concentrations will cause coma and convulsions within one minute of exposure. Certain Death.

Generally accepted physiological effects of reduced O2 concentrations.

O2 % by volume. Symptoms

reduced from 21 to 14% First perceptible signs with increased rate and volume of breathing, accelerated pulse rate and diminished ability to maintain attention.

between 14 to 10% Consciousness continues, but judgment becomes faulty. Rapid fatigue following exertion. Emotions effected, in particularly ill temper is easily aroused.

10 to 6% Can cause nausea and vomiting. Loss of ability to perform any vigorous movement or even move at all. Often the victim may not be aware that anything is wrong until collapsing and being unable to walk or crawl. Even if resuscitation is possible, there may be permanent brain damage.

below 6% Gasping breath. Convulsive movements may occur. Breathing stops, but heart may continue beating for a few minutes - ultimately death.

As it turns out, a 10 X 10 X 10 foot space, or 1,000 cubic feet, will be filled with a lethal concentration of CO2 by one resting human sized individual in just 24 hours. (The CO2 level reaches about 1% in that time). Moderate activity will cut this time to 12 hours, and strenuous activity to 6 hours.

Outside the truck, during the apocalypse, you have to assume that the atmosphere may not be breathable where you are. It might be chemical fires, or who knows what. In the case of a chemical fire, you might need to breath just a bit to die.
It might be that poisonous.

You might smell something, get a headache and then die. If you have sensing equipment, you might be able to close the vents, or if you see a toxic cloud of smoke ahead or coming your way, you could close the vents.

With a 100 pound oxygen tank, if you crack it just a bit, you could last in there for a lot longer.

I will see if I can do the calculation but you still need to scrub the CO2 or else all the oxygen won't help you.

You will still black out and die in the same 17 hours at rest, one person, in a Brink's truck sealed environment.

3 People 5 hours. Under stress, 3 hours.

So in this case I have to assume without some sort of fix, we would last 3 hours, since it will be a 3 person vehicle.

Regarding my plan to use HHO to create oxygen in a pinch, little did I know that is how they create oxygen on the space station.

"The primary source of oxygen will be water electrolysis, followed by O2 in a pressurized storage tank," said Jay Perry, an aerospace engineer at NASA's Marshall Space Flight Center working on the Environmental Control and Life Support Systems (ECLSS) project. ECLSS engineers at Marshall, at the Johnson Space Center and elsewhere are developing, improving and testing primary life support systems for the ISS.

Most of the station's oxygen will come from a process called "electrolysis," which uses electricity from the ISS solar panels to split water into hydrogen gas and oxygen gas.