On the System | Configuration page in the SA, go to the "Recordings" section and you'll see two settings for "Stop recording after how many seconds" - that should actually say "Stop recording after how many seconds of silence" but hey...
By default short messages (those less than 10 seconds) wait for 2 seconds of silence before timing out. Longer messages wait 3 seconds. You can adjust those settings here but be careful - if you crank them up too far folks will have to wait a long time for the recording to terminate... 3 seconds of silence is a pretty long time over the phone.
