Answer
Technically, it would take 44 hours (2642 minutes) for bacteria to have the same mass as earth.
Work Step by Step
In 44 hours 6x10$^{39}$ bacteria would have equivalent mass to the earth. This equation results from the weight of the earth being divided by the weight of a bacteria. Now, 6x10$^{39}$=2$^t$/20. Using this equation, we find that t is 2426. However, in the last 3.5 billion years there have been 5x10$^{14}$ generations of bacteria. Seeing as the weight of bacteria is not actually equivalent to the weight of the earth, we can see that exponential growth only occurs every couple of generations and is not necessarily consistent. Rather, when it does happen, it doesn't last terribly long. The calculation demonstrates that, when there is a lot of food and resources to feed off of, life can grow, divide quickly, and thrive. In the event there is not a lot of food or resources, they will not grow, divide, and thrive.
