How much can be recovered depends on how much of the deleted files have been overwritten by new stuff. there's no way to know for sure. But NOT using your computer helps.
The key is the fact that when a file is deleted, it is NOT deleted. Only it's entry in a "active" file index is deleted. Kinda like ripping out the table of contents pages of a book, but leaving the actual content pages alone. The space the file is STILL occupying is marked as free and the HD will start using it as needed. How and when it will get used is totally ... who knows. This is why you want to use a file recovery ASAP after accidentally deleting.
IMHO, when that "oops" happens, you don't want to shut down your computer. Don't quit apps, etc. Just launch your file recovery program.
A file recovery program "follows the white rabbit." in other words it finds a chunk of a file and follows the "markers" (for lack of the right term here) to the next chunk and so on. To the extent that consecutive chunks are in tact, the file can be recovered. And in this way a partially obliterated file can still be partially recovered.
