Formula :
$$Doubling Time = \frac{duration \times \log(2)}{\log(FinalC) - \log(InitalC)}$$
Where "log" is the logarithm to base 10 or 2 or any other base.
Units : You can use any time unit for duration. Doubling time unit will be the same.
Cite this website as following:
Roth V. 2006 Doubling Time Computing, Available from: http://www.doubling-time.com/compute.php