2010-2004 = 6 years
6 years*~365 days = 2,190 days (I don't give a shit about your "leap years")
Today: Nov 5 2010 = 309th day of 2010
OP Date: Dec 1 2004 = 336th day of 2004
Today - OP day = number of days elapsed since OP
(309+2,190) - 336 = 2,163
Number of posts over 2,163 days: 133,780
Average posts per day = total number of posts/total number of days
2,493/2,163 = 1.15256588 average posts per day
Average pages per day = average posts per day/number of posts per page (50)
1.15256588/50=0.0230513176 Average pages per day
100 pages/average number of pages per day = how many days it will take to reach 100 pages (at the current per-day average)
100/0.0230513176= 4,338.14681 days to reach 100 pages
So, since we hit 50 pages around Nov 5 2010, which was the 2,163th day of the thread, it will take 4,338.14681-2,163 days. 2,175.14681 more days. Or 5.95930633 years.
If we're half way there, then just double the time. Derp.
(of course this assumes that the average post rate is the actual post rate which is unlikely)