Introduction
Lack of oxygen itself will not be the issue
Carbon dioxide that you breathe out is the main issue

how long does it take to die sleeping in a fully sealed tent or swag?

Introduction

small tents are generally not made of impermeable walls which can seal off the air fully for good reason - you will suffocate due to build up of CO₂ if they are not vented
some winter campers will want to retain as much warm air generated by the warmth of their body as possible (see keeping warm when camping or hiking) and prevent wind chill and convective losses by keeping everything closed - but if they do they need to understand and mitigate the risks as outlined below

this assumes you do not have anything that gives off carbon monoxide such as stoves or heaters - see carbon monoxide poisoning - the silent camp killer

Don't get too worried, I have never heard of anyone dying from carbon dioxide build up in a commercially available tent or swag
- but you could get enough to cause a headache in certain situations
- and it would require exceptional circumstances such as severe frost to cause a more severe carbon dioxide build up
- nevertheless be aware of the issues as outlined at the bottom of this page
- and if you suffer from obstructive sleep apnoea or COPD then take extra care as you may be more sensitive to carbon dioxide build up

Lack of oxygen itself will not be the issue

the only times this may be an issue is if you are camping in very high altitudes or perhaps if you are also consuming oxygen with a camp stove or heater (in which case carbon monoxide is an even greater risk unless it is appropriately exhausted)
the rate of oxygen consumption = the rate of carbon dioxide production and for an adult at rest this is 0.2L/min
as the usual oxygen concentration in air is about 21%, it will take a long time for this to get to dangerously low levels of 10-14% - much longer than it would take for the carbon dioxide levels to rise to dangerous levels (see equation below)

minutely rate of fall in FiO₂ = (O₂ consumption in L/minute) / (Volume of the space in litres)

Carbon dioxide that you breathe out is the main issue

normal carbon dioxide levels in the air are neglible at ~400ppm (up to 1000ppm in occupied rooms, up to 2000ppm in poorly ventilated occupied rooms, maximum allowed workplace levels are ~5000ppm)
you normally exhale carbon dioxide at concentrations of 5% (50,000ppm) and this usually gets rapidly diluted into the air you breathe UNLESS it is a enclosed space
your body at rest usually produces 0.2L/minute of CO₂ and this will be exhaled into the enclosed space resulting in a rapid build up
inhaled concentrations of carbon dioxide at levels over 3% (30,000ppm) will cause rising CO2 levels in your body and severe headaches when you wake up, levels over 5% (50,000ppm) may kill you

minutely rate of rise in FiCO₂ = (CO₂ production in L/minute) / (Volume of the space in litres)

2P tent example

THUS, for one adult inside an average two person tent of volume 2700L which happens to be fully sealed, the rate of rise = 0.2L/min / 2700 = 74ppm per minute, and this means that in only 400 minutes (6.75hrs), the FiCO₂ will reach a dangerous level of 3% and 675 minutes (11hrs) to get to dangerous levels of 5%

If you put a Twin Zempire Monstabed into such a tent, your effective air volume practically halves and thus halves the time to toxicity!

Obviously if there were two adults exercising inside the tent, the CO₂ production would be much higher and time to toxicity much shorter!

4P tent example

let's assume it has a floor and able to be fully sealed with impermeable waterproofed fabric and the dimensions are 2.4×2.4×1.8m = 10.4cu.m. volume = 10400L
and with 3 adults sleeping the rate of rise will be 3 x 0.2L/min / 10400 = 58ppm per minute, and this means that in only 520 minutes (~8.7hrs), the FiCO₂ will reach a dangerous level of 3% and 866 minutes (~14hrs) to get to dangerous levels of 5%
NB. I'm not aware of any such tents that don't have mesh vents

most inner tents are made of uncoated mesh, untreated polyester (unless very tight weaves > 210T) or highly breathable nylon (including silnylon) which have a high enough effective diffusion rate to easily clear the 200mL/min of CO₂ produced by a human. The gas simply passes through the weave down its concentration gradient
- Air Permeability: standard uncoated tent fabrics typically have an air permeability ranging from 10 to 50mL/sq.cm/sec at standard test pressures.
- in contrast, PU coated fabrics are relatively impermeable to gas and will trap CO₂

even “breathable” fabrics can become dangerous if they become saturated with water or covered in frost

waterproof tents, bivvies or swags fully made with PU coated fabrics, Dyneema fabric or treated with DWR do NOT diffuse carbon dioxide and MUST be vented when sleeping
- this includes most polycotton fabrics used in swags as well as inner tents such as the Goout Springhill Penglai Kangaro TC bird-wing style tents as these all generally have a waterproof PU layer
- uncoated, traditional canvas swags which require seasoning generally allow adequate gas diffusion and do not need to be vented although they may still need to be to reduce moisture condensation
- for example, clear fully PVC “tents” such as this may cause a build up of CO2 especially if two people are in it during a storm with it all taped up to stop the rain seeping through Youtube - two people camping in see through clear PVC "tent" in a thunderstorm

untreated polyester fabrics with weave density of 300T or higher (or a higher than 70D thread Denier) will not diffuse CO₂ adequately and would need venting

CO₂ is heavier than air and will settle near the bottom of the tent
- 4-season solid-wall inner tents feature mesh panels placed at the absolute peak and floor level to create a chimney effect, physically forcing CO₂ out
- as a minimum to reduce heat losses and prevent CO₂ build up, vent the bottom of the tent at two ends, after all if you have to vent out air in cold weather you may as well vent out the coldest which will also be at the bottom