The duration of the New Icon can be modified using STSADM utility. Use “Days-to-show-new-icon” property to set the duration in days. If the duration is set to “0” (zero) then the new icon will not be displayed. This is the better way to hide the New Icon.
The default duration of days is “2”.
Use the following STSADM command to set the New Icon display duration to “5” days.
TechNet Reference: http://technet.microsoft.com/en-us/library/cc287681.aspx