2-3 times per week has worked best for me.

Most of the time offering something of high-value for free, then a promotion for a product of mine maybe 1 out of 5 times. And it has to be something that will complement my list well. But I do have a short 3-line blurb about my infoproduct catalog at the bottom of every email. So it's like a "By the way... I've created some great products that will help you become more successful with _____, you can find them at _____.com/catalog"