It begins to run whenever they open the gates on the cameo dam. Hence, if the gates were open now it would be running regardless of flow.
That being said they normally open the gates at 18,000. Yet, to get the truly epic play at Big Sur you will want to wait until it is 24,000 and up. It is still good at 34,000, so we don't know how high it would need to get to be 'too high' and flush out. It was great at 20,000 when everyone had boats over 9'6 for freestyle and we all carved since only a few people had planing holes. Now with the short boats and the tricks that you will want to do you will want to wait until it hits 24,000. When it does, drop everything and go.