What these folks did was attempt to crack the Twitter indexation case with a detailed report that included over 19,000 Tweets from selection of differently sized accounts. Greg Finn has written a post that talks about how this operation was done by Stone Temple Consulting, and shows us the break downs, graphs, and results of their work. You can find all the data and information on the matter on Search Engine Land by following the link below!
Search Engine Land – Study: Why, When & How Often Google Indexes Individual Tweets