So....
Here we have a graph of Dru's last 1000 posts, the raw data for which was extracted from here:
http://www.cascadeclimbers.com/threadz/dosearch.php?Cat=0&Forum=All_Forums&Name=357&Searchpage=0&Limit=1000&
We can see that the over the past couple of months Dru's posting frequency has indeed been linear, with an R^2 value of 0.97 and a posting frequency of about 22 posts per day. This means that should his posting frequency maintain its linear trend, post 20,000 should occur in 21.4 days, or roughly on the morning of Tuesday, July 6th. This does not take into account the obviously visible anomalies in the data due to periods of diminished posting by Dru, presumably during weekends.