There is no way I know of to do it automaticly, however I would suggest breaking up the search in parts so that you can estimate. See the following SO Post for details:
updating progress bar during a file search
For the first n levels of directories (say 4, including drive), count
the number of sub-directories. This is typically a quick operation,
although you can tweak it to only recurse when more than say 5
subdirectories are present or similar. Store this number.
As you perform the real search, keep track of the number of
subdirectories you've completed that are within n steps of the root.
Use this number and the stored count to estimate completion.
Essetially you will be able to estimate the % complete based on the sum of directories searched and the sum of all directories.