Average human breathes 6.6 l/min of air consuming 18.4 l/hr of oxygen and produces 16 litres an hour of CO2
http://www.aopis.org/aiha-indoor%20air%20qual. pdf
If a coffin is say a box 2m x 1m x 0.5m that's a volume of 1 cubic meter but it's probably filled a lot so say half of that
If a person is say 100kg and roughly the same density as water that's 100litres which would give you 400 litres of air or 21.7 hours.
BUT that's ignoring the CO2
http://www.esc.cam.ac.uk/ivhhn/guidelines/gas/ co2.html
says that loss of conciousness after 10 minutes results in exposure above 10% - Roughly speaking from the figures above that'd be about 2.5 hours.
Of course the breathing figures at the top are relaxed - if you're concious in a coffin that's unlikely to be the case and you might easily breathe several times that amount.
Does this help