How To Change the Duration of the !NEW Tag When Adding Items in SharePoint Lists/Libraries

To disable altogether:

#go to 12 hive
cd /d %programfiles%\Common Files\Microsoft Shared\Web Server Extensions\12\BIN

#to disable !NEW altogether
stsadm.exe -o setproperty -pn days-to-show-new-icon -pv 0 -url [Your Virtual Server's URL]

To extend:

#go to 12 hive
cd /d %programfiles%\Common Files\Microsoft Shared\Web Server Extensions\12\BIN

#to extend showing of the !NEW tag to 4 days, for example
stsadm.exe -o setproperty -pn days-to-show-new-icon -pv 4 -url [Your Virtual Server's URL]

The good news: no IISRESETs needed
The not-so-good news: it affects the behavior of !NEW for every new item/document

Details are documented in the following KB:

How to stop the !New tag from appearing when you add items to your SharePoint Team Services and SharePoint Services Web site
(http://support.microsoft.com/default.aspx?scid=kb;en-us;825510&Product=spts)