As a business owner, I'm curious about that as well. I do not email my customers on a regular basis because I don't want them to feel like I'm spam. I might email them once a quarter or so.
Personally, if it is something that I'm questionable about, I will sign up with my junk email address. If it is something I'm interested in, I will give my real address. If the emails get out of control, I will go back to the site they originated from and opt out of the emails.
If it is something I am interested in, I don't mind a weekly email. I think anything more than that is bothersome.