Short Answer: Immediately!
Longer Answer: As soon as you file your bankruptcy, the federal court sends notice to all of your creditors you listed in your bankruptcy petition. It takes a few days for them to receive the notice, but if they attempt to contact you to collect on a bill after they receive it, they are in violation of the Automatic Stay in bankruptcy. As long as your bankruptcy case stays active, they are not allowed to contact you. Once you receive your discharge, the debt no longer exists so they will not make attempts to collect on it.
Read More: Automatic Stay