The mean generation time or "doubling time" (g) is the average time required for all the components of the culture to double. This is calculated from the following equation:

log_{10} N_{t} = log_{10} N_{0} + g log_{10}2

or alternatively:

g = (log_{10} N_{t} - log_{10} N_{0}) / log_{10}2

Again, for the experiment you have just done:

t_{0} = 1.5h: N = 8.4x10^{1}, log_{10} N = 1.92

t = 8.5h: N = 3.39x10^{8}, log_{10} N = 8.53

g = (log_{10} N_{t} - log_{10} N_{0}) / log_{10}2

= (8.53 - 1.92) / 0.301

= 21.96 generations in 7 hours

= 7 / 21.96 = 0.32 hours (x 60 = 19.2 minutes)

Note that µ and g are related to each other: µ = ln2/g = 0.693/g