Quick answer: Never
Cellular service is quickly expanding across the planet. There are currently around 7 billion phones in use worldwide — which is about the same as number of people on the planet. Cellular penetration rates, however, vary widely by country. Hong Kong has 240 phones in use per 100 population. Many countries in Africa, there are only a handful of phones in use per 100 population. The United Nations Broadband Commission estimates that 57% of the world’s population is mostly offline due to access or cost.
Penetration in Sub-Sarahan Africa is particularly limited. Around 55% of the population has a phone, a figure growing by a few percent per year. However most of rural Africa has no cellular coverage.
There are two strong reasons why global cellular coverage will never reach 100%:
First, coverage is easiest where there is high population density and a prosperous population. Cell towers are generally placed 1-2 miles apart (at the least). The fixed costs of cellular infrastructure impose economic limits on regions cellular networks can serve.
Second, competing technologies (drones, balloons, satellites) will soon more cost-effectively reach remote regions than cellular networks. Since new networks will reach everywhere on the planet with no regard to geography, poor, rural areas for the first time will benefit.
So the bad news is that while cellular networks have revolutionized the lives of the majority of people on the planet, they have limits. The poorest people in rural areas will never have affordable cellular coverage. Fortunately, new technologies will address this shortcoming in the next few years.