How long after taking caster oil can you take it again I'm 39weeks pregnant?

Castor oil won't induce labour - it's a myth. It shouldn't be taken because it can cause diarrhea and abdominal pain, neither of which is particularly healthy. Your baby will come when it's ready. Pushing it to come sooner risks prematurity and the complications associated with it. Inducing labor before the cervix is ready increases the risk of cesarian section.