Merchants tend to worry about annoying customers, but people's tolerance for pain with emails is very high, especially if you deliver any kind of value or entertainment in the slightest.
My general rule of thumb is weekly.
Certainly don't spam people, but you can get into an issue where if you don't email them enough, they forget. They're like, who is this? When did I sign up for this? And you'll get more unsubscribes by sending too few emails. Some people might want to buy right away, others will need more contact. It all depends. The key is giving informative and entertaining content, with a PS of “hey you could buy this now here.”
The other approach to get comfortable with sending more email is through segmentation. That way you are optimizing your communication and not just spamming your entire list.