Over at the 1000 Memories blog, Jonathan Good poses that question and does some math to come to a reasonable answer.
Estimates suggest that it took 190,000 years for the population to reach 4 million (in a modern context that’s the number of people who ride the NYC subway each morning!). It wasn’t until the dawn of the industrial era in 1800 that our global population hit a seventh of what it is now, inching towards 1 billion. It’s rocketed ever since.
Before you click over, take a stab at a guess. The actual answer seems high to me, but then I asked a few people, and they all guess way higher. Seems like this is one of those questions that is completely without context, so any of a range of numbers could be plausible.