How long Can you breathe if buried alive?

Answer 1

Average coffin size is 84 inches; width 28 inches, and height 23 inches

which is 2.08m 0.71m, 0.58m

which is 856 liters

A part of it is filled (with you and other stuff) so lets lose 30%.

856*0.7 = 599 liters

Your problem will be carbon dioxide buildup. You can cycle air through once before carbon dioxide levels go above 5% and you have real problems. At this point oxygen level is around 16% which is enough, though not comfortable.

But carbon dioxide is making damage in your body and you will be probably dead when carbon dioxide level exceeds 7%. Quite painfully I might add.

Human consumes 450 liters of air per hour.

So you are in serious trouble in 1.3 hours and dead before 2 hours.

At-least I think so, though I am not an expert.