User names will generally expire after they've been unused for 10 weeks plus 1 week for each completed year of registration. This is counted from the last time it was connected as identified with NickServ. If the nickname you want is not in use and you want it, you can contact somebody with freenode staff to unassign it for you. Some nicks are not eligible to be unassigned, even if they're expired, but a staffer will let you know if that's the case.