save-completions-retention-time

save-completions-retention-time is a variable defined in `completion.el'.
Its value is

336

Documentation:
Discard a completion if unused for this many hours.
(1 day = 24, 1 week = 168). If this is 0, non-permanent completions
will not be saved unless these are used. Default is two weeks.

You can customize this variable.