I resorted to Google to find the answer to this question, and here’s the best answer I could find:
In ordinary English text, there are on the average about 4.5 letters per word
An introduction to information theory: symbols, signals & noise by John Robinson Pierce
I’m going to make a guess that the average is somewhat higher than that for academic writing overall, and scientific writing in particular. The reason for this is the higher lexical density of academic and –especially — scientific writing. That means more content words (nouns, verbs, adjectives, adverbs) and fewer function words (be, do, get, prepositions, articles, pronouns, etc.). Since most function words are short (2 or 3 letters), the average number of letters per word is likely to be higher.
If you have any better stats, please reply to this post and leave a comment!