About 4.5 grams. So if you're mailing a letter that can be at most 50 grams, you should be able to include about 10 sheets of paper plus the envelope. Of course, the real answer depends on many variables. Explanation below. (14 February 2006)
Resources and links on my paper A Limited-Diffusion Algorithm for Blind Substring Search, from CITSS'98; Best Student Paper award winner. (1 June 1998)