Answer
It would take about 20 hours for the cell to become the predominant in the culture.
Work Step by Step
If we use the provided equation we can determine the amount of original cells after the equation, which, calculated, is $10^6 x 2^t/20$ where t is time. This means the number of mutant cells at time t was $1 x 2^t/15$. If we wanted to figure out when the mutant cells became the dominant cells we would need to make those equations equal each other, which would be $10^6 x 2^t/15$. By taking the logarithm to base 10, then solving for t in both equations we get 1200 minutes, that being 20 hours. At 20 hours, the culture would contain $10^6 x 2^60 + 1 x 2^{80}$ cells, equaling $2 x 10^{24}$ cells. This can only be a theoretical experiment, because a single bacterial cell weighs $10^{-12}$. This would mean that $2 x 10^{24}$ cells would weigh $1 x 10^{12}$, which would be 2 million tons.
