Search notes:

SQL Developer: Tools -> Preferences -> Code Editor -> Completion Insight

Specify settings such as

Enabling of completion auto-popup in SQL Worksheet
Enabling of completion auto-popup in PL/SQL Editor
Popup Speed
Suppression of pop-ups for more than n matches
Automatic generation of column/table aliases
Enabling of semantic analysis info tip

See also

Tools -> Preferences -> Code Editor -> Code Templates