If I remember right you have got a couple of minutes per cubic feet of air in a confined space before you get carbondioxide overload. So a coffin 6x3x2 would be 36sqft. which would leave about an hour before there wasn't enough oxygen left and you started to go into carbon dioxide asphyxia. That would last for maybe another 15 to 20min before you lost conciousness and slowly your brain would cease to function in a reverse cognizance mode. So say two hours and you are dead after an hour and a half you would have severe longterm brain damage. As for thirst depending on ambient conditions the average person can live between two and five days without food and water. In sever climates (desert, arctic) your survivability drops considerably.
Cheers,
Scotsman