The CO2 problem is pretty complex because the biology behind it is complex, and because there are ways to partially help it - like the rebreathers/filters (you see those in Das Boot). And it's not something that suddenly runs out.
For example, some info dug up on Wiki:
Quote:
1% can cause drowsiness with prolonged exposure.
At 2% it is mildly narcotic and causes increased blood pressure and pulse rate, and causes reduced hearing.
At about 5% it causes stimulation of the respiratory centre, dizziness, confusion and difficulty in breathing accompanied by headache and shortness of breath.. In addition at this concentration panic attacks may occur.
At about 8% it causes headache, sweating, dim vision, tremor and loss of consciousness after exposure for between five and ten minutes.
Adaptation to increased levels of CO2 occurs in humans. Continuous inhalation of CO2 can be tolerated at three percent inspired concentrations for at least one month and four percent inspired concentrations for over a week. It was suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a submarine) since the adaptation is physiological and reversible.
The average person, through the natural process of breathing, produces approximately 2.3 pounds (1 kg) of carbon dioxide per day. The actual amount depends strongly on the person’s activity level.
Density of CO2 = 770g/L
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Therefore 45 people would, on average, produce about 2kg or 3L of CO2/hour. But how much air is in the U-boat? Someone can probably figure this out, but of course most of the pressure hull is probably NOT filled with air. But then in any case, it's a little bit hard to put a single figure on it since human activity really varies CO2 production/oxygen consumption. Then there is other stuff. Any fire would eat up oxygen at a massive rate, instantly. Chlorine gas in the air would probably reduce the life expectancy of the crew by a lot.
I will look it up, but I know that in fact some boats stayed down for a very long time. I think Jurgen Oesten mentioned an incident where a boat under his command stayed down for almost 40 hours. I think it was even possible for boats to stay longer.
By the way during this period, oxygen recycling equipment existed. It was not carried by U-boats however. For example when I toured the Russian D-2, I was told about how in 1936, the boat spent a full month on the bottom of the baltic with its full crew, testing the effectiveness of that equipment.
I think one reason Germans were not very interested in this type of thing is that batteries were by and far the greater limiting factor during this period. It was generally expected that, at least in open ocean, the sub would run out of battery long before it ran out of oxygen. At 40 hours, that's definitely something that would happen. I think the logic was that if a boat cannot surface after 30-40 hours, it probably can never surface at all. By and large that was true.