I am going to offer a slightly different perspective, and say it varies for each audience segment. In other words, there are some companies that I want to hear from more often, and some I don't. The challenge, and what Marketo can help you with, is segmenting your users, and sending the most optimal number of communications for each user.
You can create smart lists in Marketo based on engagement (email open and click rates). For example, if a group of people has high engagement then send them more emails. If the converse is true for another, send this group less emails.