Using the formula, we would have
T = 70 / 3.2, or 21.875 years
I'd say the approximate value is 22 years, rounding to the nearest whole year.
For the math behind the "rule of 70", please see
Note that instead of the "rule of 70" some people use the "rule of 72", for example
Either way, we get the same approximate value of 22 years (when rounded to the nearest whole year).