Well I can extrapolate to a degree from logs and under-estimate to keep it relatively accurate.
Assuming each message or /me has 10 words in it, each page of logs has 10 messages (/me's or server logs), and from pg 26 of logs, to pg 500, just under 2 days have passed, let's assume the daily word count is around 25,000 words (Realistically it's definitely higher - but undervaluing the daily word count can help us account for slow days/days off)
5 RP days per week, so each week, 125,000 words, estimated.
52 weeks in a year, so 6.5 million words per year, again, estimated, not accounting for downtime. There's probably a lot more words per year in reality.
We've been up for, what, 3 or 4 years? I forget.
Let's say 4 years - 26 Million words. At the very least. For main server alone.
Realistically though - I'd probably say somewhere around 50 million, probably more.