You're assuming that there's something more to disclose.
The kind of disclosure that everyone talks about having happened in the UK etc. as of late actually happened in the US many years ago, when the Project Blue Book files were made public. You can view many of them on-line at the Blue Book Archive (I'm not sure how many are available these days - it's an ongoing project), or you can get them all on microfilm.
The point is, the US government has been as forthcoming about its UFO investigations as any other government. Indeed, I would argue that it was ahead of the curve.
Now, if by disclosure you mean disclosure of an alien presence, or something like that, then I would say zero, first because there's no solid evidence to support the assertion that the government is keeping that kind of secret, and second, if they really are keeping that kind of secret, it could only be done with the acquiesence of the aliens, which means that the "truth" wouldn't be disclosed until the aliens want it to.