Two billion years, and it seems to be quite the fluke. One kind of single-celled organism ingested another, and instead of killing it or being killed, it provided a support system and in return got more energy back. This was a one-time thing that changed the course of evolution, making multi-cellular life possible.

Question is how often does that occur on other planets with life? Maybe Earth is unusual in that life sticks around for long enough that low probability events happen. Maybe it happened far earlier than it would normally take. The universe is big, low probability events will happen.