From my own personal experiences, I would provide a ballpark estimate of around 50 to 100 words per post, on average, which would cause the total to be between 1.25 and 2.5 million words in total.

I suppose that there could be tools that could assist to scrape that kind of data, as ImThour is suggesting.  By the way, some of my posts contain my citing of other posts, so I would speculate that if I am citing someone else then the words contained within those citings would not count towards my overall word count.