Basically its what ever the controller or FISO has asked you to call.
It could be base turn complete when you start pointing towards the airfield on an intercept if your rubbish (it is actually quite common to get it right and intercept as you roll out) or when your actually established.
Which one they ask you for tends to depend on what you are doing and if you have been cleared to decend with the procedure or not.
Some places they won't clear you to decend with the procedure until you report established. Some clear you for the procedure report base turn complete. Just so they know where you are.
You might think its all a pile of rubbish but some places when you call base turn complete that's when they chuck all the snowploughs etc off the runway and if you don't you will end up having to do it again.