About how long could you stay underwater?

In a previous post I wondered how long you could stay underwater breathing the air left in your capsized kayak. Now the reasons for wanting to do this could include: scaring your mum; performing tricks like swapping kayaks underwater with a friend and then rolling up; and avoiding surface problems – like giant pterodactyls or maybe some extreme weather?
Anyway I did the math and here is what I think. The overall volume of a kayak like mine is maybe 380 litres – this is a guess based on these kayaks. Now minus the bulk headed areas fore and aft of 31 and 84 litres respectively (from here) equals 265 litres. But its got some water in it right? Lets say its a sixth full of water (although if you went straight over then wet exited I think it would be less) i.e. 44 litres of water and 221litres of air. Now according to this site (a fascinating story about Buteyko’s theory and of why asthma research focuses on drugs when it should also be looking at breathing) a healthy adult at rest breathes 12 times a minute and inhales 5 litres of air. However maybe you would be a bit stressed out if a pterodactyl was circling overhead? Lets up that to 20 times a minute and 8 litres of air (from the same site for hyper ventilation rates). Note: to keep at even this increased level in these circumstances would require great self control. So 221 litres of air divided by 8 per minute = 28 minutes. However there are two further variables. Firstly the lungs don’t use all the oxygen in each breath. The human body under average conditions inhales air which contains approximately 21% oxygen and 0.03% carbon dioxide. The air breathed out of the lungs contains approximately 15% oxygen and 5.6% CO₂. So you use the same oxygen a couple of times? Or can you? In fact the second problem is the growing concentration of carbon dioxide would eventually reach poisoning proportions. Basically it starts with labored breathing, headaches, clumsiness, loss of energy at 1-2% then descends into respitory paraylsis, unconsciousness and death 10%. (Here is a great paper titled Carbon Dioxide Caves and You). So how do I find out how long 221 litres of air would keep me alive? Its clearly not 28 minutes… How long will have to wait. Perhaps if I have time I will find the answer to this question but for now its time to read my book. If you want to read more how about here, another interesting side note about concentrations of CO₂ or here