I think it was 3 days by default, but i am not an 100% sure.
But ofcourse you have access to the stsadm tool. It is installed with sharepoint, and is used to set all kinds of properties. Youll find the exe in you installation dir for sharepoint (C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\60\BIN)
To run open a CMD window and chance the directory to that install folder, then you can call the stsadm.exe with its switches.
-help : list of all operations
-o : operation (setproperty / getproperty)
-pn: property name (in your case the: days-to-show-new-icon)
-pv: property value (example has a 0 so it doesnt show the icon you can change this to any number of days)
-url: (optional in some cases) in this case you specify the virtual server for the property to be set (in it contains spaces use ""s)
stsadm.exe -o setproperty -pn days-to-show-new-icon -pv 0 -url [Your Virtual Server's URL]
Its realy not rocket science...
Greetings