The time constant for a RC circuit is given by?

In an RC circuit the time constant is found by R x C. T = R x C to be precise.

It is the time required to charge the capacitor through the resistor, to 63.2 (≈ 63) percent of full charge; or to discharge it to 36.8 (≈ 37) percent of its initial voltage. These values are derived from the mathematical constant e, specifically 1 − e − 1 and e − 1 respectively.