How Do I Calculate The discharge Time of Capacitor with a resistor as a load?
If the capacitor is charged to voltage \$V_0\$ it will take 

\$R C\$ (R in ohms, C in farads) seconds to discharge to \$V_0\over e\$  \$\approx 0.368 V_0. \$

It (in theory) will never get to 0V. 

If you are timing it, you will get diminishing returns by triggering on much less than about that number as the curve flattens so your reading will be more sensitive to noise.