Some will boldly state appropriate intervals, but I haven't a clue how anyone figures they can put out a blanket frequency rate mailing list for all lists, across all markets. The fact of the matter is - there is no one size fits all for every email list, but here are some guidelines... Stay in the Mind of Your Prospects & Customers Your goal in email marketing is to keep your business in the minds of your prospects and customers.
You also want to your readers to consider you the go-to source for information you share and the products you sell. If you email to infrequently, that could become problematic as they'll just forget about you. But what mailing list about the other side of the coin? Can you email too much? Maybe, but what works best for your market and your list is up to you to test. You can look at open rates, response rates, etc. to help you determine what is best for you. Personally, I email my Internet marketing list 2-3 times per week.
It's quite likely that some people don't like it, but I've found this works very well in the grand scheme of things and I'm here to please the majority of my active subscribers. If you're on my list, you know that I tend mailing list to write short emails. And amongst the product promotions I do, there is plenty of free stuff, information, etc. In other markets I've dabbled with, I've also found a similar formula works. Stay in contact a couple times per week, keeping it short and sweet.