ido-auto-merge-work-directories-length

ido-auto-merge-work-directories-length is a variable defined in `ido.el'.
Its value is

0

Documentation:
Automatically switch to merged work directories during file name input.
The value is number of characters to type before switching to merged mode.
If zero, the switch happens when no matches are found in the current directory.
Automatic merging is disabled if the value is negative.

You can customize this variable.