org-outline-path-complete-in-steps

org-outline-path-complete-in-steps is a variable defined in `org.el'.
Its value is

t

Documentation:
Non-nil means complete the outline path in hierarchical steps.
When Org-mode uses the refile interface to select an outline path
(see variable `org-refile-use-outline-path'), the completion of
the path can be done is a single go, or if can be done in steps down
the headline hierarchy. Going in steps is probably the best if you
do not use a special completion package like `ido' or `icicles'.
However, when using these packages, going in one step can be very
fast, while still showing the whole path to the entry.

You can customize this variable.