"The product of R in ohms times C in farads is called the time constant (also called the RC time constant) of the circuit and is the time in seconds required to charge the capacitor to 63.2% of the applied voltage. (The lower-case Greek letter tau, Τ, is often used to represent the time constant in electronics circuits.) After two time constants (t = 2Τ) the capacitor charges another 63.2% of the difference between the capacitor voltage at one time constant and the applied voltage, for a total charge of 86.5%. After three time constants the capacitor reaches 95% of the applied voltage, and so on. [..] After five time constants, a capacitor is considered fully charged, having reached 99.24% of the applied voltage. Theoretically, the charging process is never really finished, but eventually the charging current drops to an immeasurably small value and the voltage is effectively constant." [2016 ARRL Radio Amateur's Handbook, page 27 of chapter 2]