How much remorse do you have to have before you may know that your sins are forgiven?

Luther:

When you build forgivenmess on remorse, you build on sand. Then you make faith in God dependent on the works of men. Forgiveness is not at all based on remorse, but on faith in Christ. He has promised that ‘whatsoever thou shalt bind on earth shall be bound in heaven; and whatsoever thou shalt loose on earth shall be loosed in heaven’ (Matthew 16:19). So, when the priest absolves you, you are free, as sure as Christ himself has promised it.