How often should I tweet?
blog.bufferapp.com
blog.bufferapp.com
Now, if the tweets do not show up in my timeline it's different of course. A company can tweet every single minute on Twitter if they reply to people and start conversations since this does not show up in everybody's timeline. See Hipmunk for a company that gets this right, for example.
Are people really judging people on how often they tweet? If you have insightful things to say, or are contributing with new and interesting posts then it should not matter about how often you tweet.
The bigger the water hose != better the twitter account
In terms of too much noise, you can always filter users and content with lists. I am sure twitter is working hard on allowing users to create streams off their raging rivers.
What do you actively use Twitter for?