I think it might be better to make it mandatory. With electronic means, it is relatively easy to make sure that people no longer wanting to receive messages can just click on a button at the bottom of the text to say so. In this case, a 10-day deadline seems like more than enough.
However, since this is not mandatory, it is not done that way. In that case, the delay might be prolonged for all sorts of reasons. Not all businesses have the same capacity to manage these situations.
What do you do then? Give them 30 days or make the electronic mode mandatory?