I here from lots of people that think they are pestering their list. Do not forget the reason that you have a list is to help your subscribers fill a need that they have. They signed up. You may have offered them something that they wanted but the pressed send first.
Your part of the bargain is to offer them a mixture of free and paid high quality information. You have to SELL something to your list if you are going to make any money. To succeed online you must make sales. Selling is how the world goes round.
No matter you offline job you are in sales in some manner or another. You might just empty the trash or do the dishes. But someone up front is selling a lot of chicken nuggets.
In this business not only do you have to sell yourself, but you either sell your own products or someone elses.
How often you mail varies is up to you and the Niche you are working in. I mail on a different schedule for my IM list then I do my Weight Loss niches.
Sometimes it is just the quanity of information that I have to cover. At times it is just the amount of available products.
But that is something that you have to decide what is right for your list. Just make sure that you are offering them good information.
But I never mail more then once a day nor do I ever send out a product to my list that I have not bought or reviewed. (More on that later).