What is the latest I can have my baby? It is overdue and I want natural birth.

Be aware. The chance of stillbirth of an otherwise normal baby increases in post mature babies. The placenta is a time limited organ where blood vessels begin to self destruct as time goes past term. It can function well when there is no pressure of the womb muscle, but fail to adequately nourish baby in labor. The longer you go, the less likely a normal labor. Pay attention to your OB's advise.
Naturally, 43 weeks. At 43 weeks most ob's will have run tests on the baby and will want to induce labor by 42 weeks.