Sitting in a closed room filled with regular air - for how long will the air last?
Since you are considering to meditate in the room, without any excess of activity, you can consider the average resting O2 consumption, which is C = 3.5 mL/(min.kg) [1].

If we ignore the volume of your body, initially the room is filled with VO2(t0) = 0.21 * 5 = 1.05 m3 of O2.

With you breathing in the room, you can consider the volume of O2 in the room as a function of the time:
VO2(t) = VO2(t0) - C * M * t
where M is your weight in kg, and where C is converted to m3/(min.kg).

An oxygen defficient atmosphere is an atmosphere for which oxigen content is below 19.5 % of the volume [2]. An oxygen defficient atmosphere can be dangerous for your health. So in your case you need to find the value of t for which VO2(t) becomes inferior to the threshold Th = 0.195 * 5 = 0.975 m3.

So the maximum time you can spend in your room would be:
tmax = (VO2(t0) - Th)/(C * M) = 335 min = 5 h 36 min

Of course, this value is a bit surestimated, and you should leave the room before that time.



[1]  M. Kwan, J. Woo &amp; T. Kwok (2004) The standard oxygen consumption
value equivalent to one metabolic equivalent (3.5 ml/min/kg) is not appropriate for elderly people, International Journal of Food Sciences and Nutrition, 55:3, 179-182, DOI:10.1080/09637480410001725201

[2] https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&amp;p_id=12716