Calculate volume of a vessel assuming choke flow discharge

I would like someone to help me to develop an equation (calculus integral) to determine the volume of gas in a vessel knowing the time to blow it down and assuming choke flow discharge. Knowing the time it discharge being 1 to 2 minutes and using a choke flow equation of Q (rate) = constant * pressure / (temperature * S.G)^0.5 where C = 1.524, P psia (declines from 500 psi to 0 psi), Temp =550 R; S.G. = 0.60. Q equals rate of Mscf/d. The rate is dependent pressure in the vessel which declines as the gas is released. I know the starting pressure and it takes 2 minutes for the pressure to go to zero. The pipe is 1/16" that give the constant of equation above. Solve for the volume Mscf.