Standard for most situations. Can change mostly because of progression of the enemy team delta XP or if your supp does a **** job which brings you behind in terms of gold and XP.
Varies also depending on the amount of magic damage dealt by the opposing team. Disable power may force the BKB right after the morbid mask for example.
In general stay shy until level 6. Last hitting is crutial, but not dying is mandatory (like for every carry in early game).
Earthquate can be chooser at level 3 to 5 (early) so it can help finish kills. Depends on the opponents agressivity and mistakes. Otherwise EQ is setup after overpower and fury swipes are well fed.