How much time does AES decryption take? I am attempting to decrypt a 90 GB file that is encrypted using AES-256. I have the password. How long should this take to complete? I've had the process running for over 3 days with no progress.
I am attempting to decrypt a 90 GB file that is encrypted using 256
  AES. [...] How long should this take to complete?


If the AES implementation and the program doing the decryption are any good this is only bound by the speed of your drive, if your processor is newer than from about 2012. So for an SSD, you should be able to hit 200MB/s and with an HDD 50MB/s with little problems, if the program doesn't have some horrendous overhead and utilizes a sane implementation and with an older / weaker processor, you should still be able to saturate the HDD and run the SSD at about half speed.

Note however that certain platforms, e.g. standard python don't actually compile to machine code and thus are extremely slow, ie in a measurement I did recently I hit about 1MB/s with the interpreter. But even then it should "only" take you 25 hours. So chances are the program you are using is extremely poorly implemented and you should strongly consider using an alternative if you can.