How long does it take to decrypt a hard drive with FileVault?
It depends.

There's several factors that go into decrypting a drive:


The CPU - how quickly it can "crunch" the numbers to encrypt/decrypt
Whether or not your have a T2 chip
Drive size - bigger means more things to process 
Type of drive - SSDs are fairly quick whereas mechanical drives are much, much slower.
How it's connected - USB 2.0 (480Mb/s) versus USB 3.0 (5Gb/s) versus SATA II (3Gb/s) versus SATA III (6Gb/s).  All these different speeds factor in how quickly the decryption process goes.


When I had to do DoD wipes of mechanical drives, many of them were only 250GB, it could take up to 48 hours and it was just a "3 pass of writing zeros".


  So I left my iMac on sleep mode overnight,


When you put your Mac to sleep, it stops the processes.  Don't put it to sleep while decrypting.