ENFORCETIME sets how long (in seconds) after someone attempts to use one of your nicks they will automatically have their nick changed.
Note: This can be set to DEFAULT to return to the network default or a number value between 1-180 (seconds).
/msg NickServ SET ENFORCETIME DEFAULT
Note: at the end of the command where it says DEFAULT you can put numbers such as 30 or 90 what ever you want. Also no that those numbers are seconds