This article talks about people spending about 2 seconds to decide whether to read or delete your email. W: http://www.smh.com.au/articles/2004/06/14/1087065076891.html.
The article refers to Jakob Nielsen's 2nd Edition report titled: Email Newsletter Usability:
127 design guidelines for subscription interfaces, newsletter content and account maintenance based on user research.
You can read the summary of the report, and the table of contents of the report at: http://nngroup.com/reports/newsletters/.
The full report costs $298 (U.S.) for the 293 page pdf file.