@nonprofitorgs polled Twitter users to gauge "How many Tweets do you think nonprofit organizations should send out per day?"
Out of 331 votes, 136 or 41% thought it should be just 1-2 a day. Another 128 or 39% thought non-profits should tweet 3 to 5 times a day.
I voted for 6-7 times a day, one of the 33 or 10% who thought so.
My thinking is that people all over the world use Twitter, as do people working in all US and Canadian time zones, and people working at all hours of the day and night. In terms of visibility, it is smart to identify your target audience and gear the number of tweets to optimally reach that audience.