Rescue breathing should be continued until the victim can breathe again or until medical help arrives.

A. Until the victim can breathe again

Explanation:

If medical help arrives and you are doing CPR don't stop to let them do it, you keep going (if your doing it correctly) because their life is hanging on a thread and even one second of time wasted can cause lots of problems.

Also if you don't know how to then try your best until medical help arrives because they are professionals.

Ya Im done talking