In the context of using twitter to directly market your products or recommend products to others (where you get a commission via an affiliate link) you should tweet realistically no more than once per day. In addition to these potentially commercial tweets you will also be posting regular updates on what you are doing.
If you consider your followers on twitter to be your marketplace (prospects) then building a relationship is the key part of the process. As people we buy from people we know, like and trust within out communities locally and the online communities work in pretty much the same way.
As a general guide you should post five tweets before you post anything commercial and then update at least five times before the next commercial message. In some cases this should be increased to ten tweets if you notice an increase in the number of people who stop following you.Respond to your marketplace.
Twitter has the advantage of not being email and therefore not requiring double opt in processes to build up a list.Followers opt to follow by pressing the button and thats the process.
The userbase of twitter is interested in a variety of things, so post variety.It is also demographically mainly male and 40 so post things that are interesting to that audience and you should do ok.
As of today(May12th) I have a click through rate from twitter of 2.3% which is something I intend to build upon.