How long "!New" image stay with newly added file

The default is 2 days (or 1 full day plus time until midnight the next day)
To change the value (you must have admin access to the server):
  stsadm -o setproperty -pn days-to-show-new-icon -pv Number_Of_Days -url http://Your_Server_Address
To check your current value:
  stsadm -o getproperty -pn days-to-show-new-icon -url http://Your_Server_Address