Anybody have an informed opinion on how long it might take an fbi expert to recover deleted files off a computer?
Depends if they've been overwritten with new data. When you delete a file most operating systems just mark that bit of storage available for re-use, and mark the filename as deleted, which really means 'dont show this file in listings'. The actual data is still there untouched.
If the file has not been overwritten, there are programs that will scan a disk in minutes and extract the missing files.
If the data has been overwritten, there are techniques for sensing the incredibly tiny magnetic changes left from a few previous data writes.
The answer is minutes to days,
if at all in multiple overwrites.