RC Time Constant

In an RC circuit, the value of the time constant (τ) is equal to the product of the circuit resistance (R) and the circuit capacitance (C). It is the time required to charge the capacitor, through the resistor, to 63% of full charge; or to discharge it to 37% of its initial voltage.

tau =RC

R=frac{tau }{C}

C=frac{tau }{R}

More info: http://en.wikipedia.org/wiki/RC_time_constant