When will you be able to start performing your duties in this position?

The answer to this question depends on your current situation and readiness. You can respond approximately as follows:

"I am ready to start performing my duties in the position immediately after completing all necessary formalities and handing over current tasks. If required, I can start within a week from the job offer."

If there are specific restrictions, such as current commitments or the need to serve a notice period, it is best to honestly communicate this, indicating approximate start dates.