 We reckon we've seen around 20 flies in the few minutes we've been looking. But clearly this isn't going to get us very far in answering the question of how many flies have ever existed. So what do we know? We've established that there are 17 million flies for every human on Earth today and flies have existed for 250 million years. But there are yet more factors to go into our calculation, like whether to count flies at every stage in their life cycle.