What is the number of years it takes a specific population to double in size?

The time it will take for a population to double is based on numerous factors, not solely time. The fertility rate, food production rate, security (from foreign invasions and domestic violence or revolt), mortality rate, and numerous other elements are requisite to determine how long it will take a population to double in size. In many cases, the combination of these factors will not lead to any growth at all (such as in a war-torn country with massive human flight or a stable country where the fertility rate is less than 2.0 and there is no Immigration).