If you know how to calculate the charge rate, then you know how to calculate the discharge rate, as the equations are the same; they depend on the capacitance and the circuit resistance. In your circuit, the capacitor discharges through the combanation of the 515 and 1K resistors. Thus, one time constant ( the time it take for the capacitor to discharge to 63% of it's voltage ) will be 550*10exp-6 * 1515 = 833mS. The voltage at that time will be (1-.63)*V2. It looks to me like the light will fade out in just under 1 second.