I have a client who wanted to control how long an item will be shown as new in SharePoint List. Later scrutinizing I have found that it's an configuration issue. There's nothing development required. I have got the link helpful http://my.advisor.com/doc/17728
The !New icon displays next to a new list item in a list or document library for a defined amount of time based on the "days-to-show-new-icon" virtual server property. You can modify this property using the operation setproperty in the stsadm.exe command-line tool. For example, this command sets all occurrences of new list and document library items in the http://wss1 site collection to display the "!new" icon for two days:
stsadm.exe -o setproperty -pn days-to-show-new-icon -pv 2 -url http://wss1
To remove the !New icon all together, simply set the –pv value to 0.