It updates at 5 minute intervals, as does Apple's coundown timer in the pink section to the right on this page:
http://www.apple.com/itunes/100million/
I noticed that from the last interval before 97.5 million to the first interval over that, the change was 2968. That 97.5 million is a number divisible by 100 thousand between 95 and 100 million, so people do make submissions to attempt to win an iPod. But I did some recalculating, in which I took the finite change of the number of entries from 12:57est. to the most recent update at 14:57 (the count then was 97 552 402, about halfway between one hundred thousand markers), and divided that by the finite change of 120 minutes. In mathematical terms, I did the following
avg. change in entries (E) = ∆E/∆t
I came up with an avg. 436.108 new entries per minute. That means for every update interval of 5 minutes, on avg. during 12:57est and 14:57est, 2180.54 entries were added. That number is only 73.468% of the change in the number of entries from the last interval before 97.5 million to the first interval over 97.5mil.
This, I conclude that the rate in which the number of entries submitted around every one hundred thousand increases. So if you're going to do the tell a friend, be quick.
BTW: Doctor Q, you're not the only one doing the math around here.