Hired new helper, what happens to current?

We hired a new helper from overseas and will be working for us in 3 weeks (after quarantine).

We are breaking the contract with our current helper.

Question:

When does the "timer" start for our old helper to start finding a way home or new employer? Does it start when we give her the date on the termination notice.....or does it start when our new helper gets her Visa stamped? I dont think we can have 2 helpers at once, so I figure when one comes in, the other is getting let go in Immigrations eyes?

Thanks