Update using PowerShell
http://spsawyer.wordpress.com/2013/02/21/mysteries-of-the-new-icon/
Update New Icon using STSADM
The * NEW tag appears in lists and libraries for 2 days by default, but due to timing may last three days.
STSADM is an easy way to configure this setting. There are several references to this on the web but just wanted to include the code here and show how easy it is to configure.
First start by running SharePoint Management Shell and run the following
stsadm -o getproperty -pn days-to-show-new-icon -url http://<web_application_Server_Address>
This will display the number of days that the *NEW icon is set to appear.
where pn stands for propertyname
Did you get an error? If so note that if you need to elevate your credentials hold down the SHIFT key and right click the SharePoint 2010 Management Shell Icon.
To turn off * NEW add the following
stsadm -o getproperty -pn days-to-show-new-icon –pv 0 -url http://<web_application_Server_Address>
where pn stands for propertyname and pv stands for propertyvalue
Turn *NEW off for the http://intranet.contoso.com web application.
The change was instant and effects the entire web application.
To turn off * NEW add the following
stsadm -o getproperty -pn days-to-show-new-icon –pv 0 -url http://<web_application_Server_Address>
where pn stands for propertyname and pv stands for propertyvalue
Turn *NEW for 2 days for the http://intranet.contoso.com web application.
Note that if you attempt to set this settings at a subsite level, you still set that settings for the entire site.
References
! New Tag Info
http://office.microsoft.com/en-us/windows-sharepoint-services-it/configuring-discussion-boards-HA001160789.aspx
http://msdn2.microsoft.com/en-us/library/ms462807.aspx
http://support.microsoft.com/kb/825510 (old)