No matter where you are in the world, no matter who you are working among, no matter how many believers you have, it will typically not take more than 17 “doublings” of the Christian population to saturate most districts.
20 to 25 doubles would suffice for nearly any province. 32 doubles would see the entire world Christian–even if you are starting with just one person.
Early doublings can be easy. Later doublings are hard.
The biggest question in terms of when the world will be reached is–how long does it take for a given group to double in size?
If it takes a year to double, then you’re looking at, minimum, about 20 years to saturate an area (provided you maintain speed). If it takes 20 years to double, then you’re looking at 400 years.
The longer it takes to double, the more can happen “in the meantime” to prevent the doubling from ever taking place. Speed of doubling is your ally. Anything that slows you down increases the risk it will never happen.