When Minecraft gets acquired for 2.5 billion by Microsoft. Anything before that isn't a billion dollar endeavour. It could have failed miserably. It could have gone nowhere. 0.0.1a was straight up crap, and barely more interesting than Infiniminer. It also hadn't sold for a billion dollars worth of copies either, so...
A business is worth a billion dollars when it either has made it through sales, or someone pays you for that amount. VC funds don't even count, as the vast majority of billion dollar VC funded companies are money hemorraging holes.