When are we going to finish this thing?
A. By current trends, we aren't!
First, the question implies the task of the Great Commission can be finished. This is the missiological concept of "closure." Not everyone agrees it can be done, theologically speaking.
To me, however, the theological issues of closure are moot: it won't happen any time soon, because we're not going in sufficient numbers to the places who have not yet heard.
The Status of Global Mission 2014 shows how the church has pretty much been stagnant (globally) at about 33% of the world since 1900 (i.e. for 115 years). Furthermore, while the world's % unevangelized has dropped from 54% in 1900 to 29.3% today, the absolute number of people who have no access to the Gospel rose from 1.8 billion in 2000 to 2.1 billion today, and will likely reach 2.3 billion by 2025 and 2.6 billion in 2050.
The finish line is running away from us faster than we are running toward it. We can't finish the task when we aren't making measurable progress toward the finish line.
Why aren't we making this progress? Because we, for the most part, are not engaging with non-Christians. Researchers estimate 86% of non-Christians (mostly Muslims, Hindus and Buddhists) do not personally know a believer.
In Acts 1, Jesus told his disciples "you will be my witnesses" -- the individual believers. Set against this is the fact that most non-Christians do not have this witness in their life. We try to leave it to professional church programs, media, Bibles distributed as "silent witnesses," tracts "left for some seeker" in various places, and the like. But the fact is, WE are to be the witnesses.
Should we actually engage this task amongst those who have no witness (because they do not have us), I would then point out that another element - we have to be a witness both now and in the future: today's generation has the opportunity to be discipled--and future generations do, too. I've covered this under sustainable closure.
But I worry "less" about future generations when we're not doing even enough to reach the present one. When a student can't even add 3 and 4 correctly, you worry less about teaching them to multiply, and forget about the higher concepts of algebra and geometry...