Well, the only number we don't have that we need is the volume of this air-tight room in which you are going to survive.  Since you didn't provide one, I will give a sample.  How about a room that is 10 meters long, 10 meters wide, and 10 meters high?  To calculate the volume of that room, we would multiply the length times the width times the height, which would be 1000 cubic meters.  To get a comparable unit, you said the volume of air in an adult is about 500 milliliters;  a cubic centimeter is equal to one milliliter.  There are 100 centimeters in a meter, so to get the number of cubic centimeters in a cubic meter, we would multiply 100 times 100 times 100, which would equal 1,000,000 cubic centimeters in one cubic meter.  Since we have 1000 cubic meters, that times 1,000,000 would equal 1,000,000,000 (1 billion) cubic centimeters of air.  That is the same as 1 billion milliliters of air.  If one breath is 500 milliliters, we can divide by 500 to get the number of breaths that volume would represent, which would be 2,000,000 (2 million).  If you breathe at a resting rate of 18 breaths per minute, we divide the 2 million breaths by 18 to get the number of minutes available for you to survive, which would be 111,111.  To get that to a more recognizable number, there are 60 minutes in one hour, so divide that huge nuber by 60, which would give you 1851 hours.  There are 24 hours in one day, so divide again by 24, and you get 77 days.  You could exist in that room, breathing-wise, a little over 2 months!