You can send your newsletter as often as you like...as long as you don't have more than 2 people unsubscribe for every 1,000 emails you send (.2%) . If you lose more than 2 people out of every 1,000 emails, then you're either sending too often, or you've strayed too far off topic (such as discussing politics in a newsletter where people were expecting art). I sure wish I could remember where I first read the 2% number cited, but I can't, so I did some quick research, and came across these statistics from a 2010 Direct Marketing Association research paper that reports - those who email to their own "house" list have an average unsubscribe rate of .77%. So let's say, based on that research, we'll set an upper bound of 7/1,000 unusbscribers .
What I'm recommending is this: for every 1,000 emails you send, you pick a number you can accept to lose...it can be as low as 2 or as high as 7 - but it can't be zero (zero is unrealistic once your list grows past a certain size). Incidently,FineArtViews shoots for .2% (our current unsubscribe rate is .04% (4 out of 10,000)).
How to Measure Your Unsubscribe Rate
First of all, you have to know who unsubscribed.  Yes, that means you cannot and should not send your newsletters with your regular email program using the BCC field for all your subscribers. Seriously. Don't. Do. That. What you need to use is a tool made for sending email newsletters like MailChimp, Constant Contact, or Aweber. If you're aFASO customer, we provide an email newsletter manager automatically integrated with your website (and Facebook!) that tracks all this stuff.
The whole article is here.