Note:
The default duration of days is “2”.
Using STSADM
Use the following STSADM command to set the New Icon display duration to “5” days.
stsadm.exe
-o setproperty
-pn days-to-show-new-icon
-pv 5
-url http://mosssiteUrl
TechNet Reference: http://technet.microsoft.com/en-us/library/cc287681.aspx