This is a bit of a loaded question because it completely depends on your audience and the type of content you're sharing. For example, sharing articles and info pieces several times a week may be ok, whereas sending several "buy me" type emails in one week is likely off-putting to your audience.
Do you have an email cadence that has previously been established that you are trying to modify or optimize? That would be a good place to start. If you're starting fresh, I'd suggest sending 2-3 emails/month and ramping up from there. With every additional email you add, watch your metrics and see how it's effecting open and click rates among whatever you've established as the end goal for these communications, i.e. lead gen, registrations, sign-ups, etc.
In either case, we like to follow the Jab, Jab, Jab, Right Hook methodology of sending info/value added content three times before a product/sale-specific communication.
Hope this helps!
Groupon did an amazing job of determining this. They simply asked their subscribers how often they wanted to hear from them. Imagine that.
At least then when you sign up for daily deals, and realize its too many emails, that you were the one that picked that and instead of unsubscribing you just change your preferences. I thought it was really well done.
I agree with what others have said above. As long as it is useful insightful content, people didn't unsubscribe in droves. Even if I sent one email and it was very sales, buy my product, I had a huge spike in unsubscribes. I think its less about the cadence and more about useful content. If it makes them look smarter to their boss and helps them in some way, that goes along way.
I agree with everyone here - it depends on the audience and also on the content.
To Pierce's point - ASK! Or at least make different options available (think preference center). You will start seeing trends and be able to put the best cadences in place for your company.
If you are promoting multiple products or services, you can set up an email preference center to let the recipients decide how often and which emails they want to receive.
I agree with a lot of the comments about asking people and this depending on the audience. I would also run some simple tests - try different approaches and let the data give you the answer.