Based on an article * published in 1961, there exist in average 4.5 characters in each English word. Evidently, in 52 coming years, this number has increased by inventing new longer words (like propositionalization). With no reference, we assume 6.5 characters per word in 2013. As we have only 50% confidence to our convention, so we take the average and we end up with 5.5 characters per word. Based on this induced number, a tweet with maximum number of characters (i.e. 140) has potentially 21.5 words **.
* An introduction to Information Theory: Symbols, Signals and Noise by John Robinson Peirce
** We also consider one space per word.