From Steemit FAQ:
How often can I post?
You are allowed to post almost as often as you like. Currently, posts must be spaced 5 minutes apart. However, the community may not find value in users that are posting too frequently. Keep in mind what your audience will be interested in viewing, so that you do not overwhelm your followers with too much content.
In short, the community determines that. If you post less frequently, you will have less reward. But if you post too frequently, you may face negative feedback from the community.
I agree with your point. I personally feel around 2-3 times a week is solid.