I think a lot of it depends on what you are promoting. I have been using email marketing for a few years now and I send my list out when I have a new product our group of products to promote.
I do not want to flood my site visitors with emails pushing the same items. I tend to protect my list and do not want to abuse the members that have given me an email address. I normally have some type of update about every 4 to 6 weeks and send that out to the members. Sometimes a little sooner or later depending upon what I have available.
But again I think a lot of it has to do with product. My products, which are training products, take a long time to develop so that has a lot to do with the frequency.
Now a few times a year I will have special offers on existing products, or package sets that I have assembled based on feedback from my site members and those will have a special email go out. I try to limit those special offers to 2 or 3 a year. Again I do not want to annoy my site members. I may be a little over cautious but I would rather error on the side of caution than annoyance.
I will say that this approach has worked well for me as my unsubscribe rate is very low, usually less than 1% a year. I wish my conversion rate was a little higher, but I think all of us have that same wish... Right now my conversion rate stays right around 4 to 5 %.