How long would it take for FileVault to encrypt my Retina Macbook Pro?
The process to enable FileVault will read the entire 500 GB of data - whether the block is empty or full and encrypt it with the keys you set up as part of the process. So, the background IO will run the fastest if you don't have other user level disk IO running. The encryption itself will take less than 10% of one CPU on that powerful (fast) Mac - so you are really just going to see a sustained 60 to 80 MB/s re-write of the entire drive if you let the Mac sit idle. A Mac with a spinning hard drive would see between 20 to 30 MB/s so an Air or any Mac with solid state drives will be two to three times faster in this operation.

So - from the time you start, I would estimate 2-3 hours if you are getting at least 70 MB/s for writing the encrypted data back to the disk. This process does run in the background and isn't really reversible once it starts, so you can kick it off and then track the progress with diskutil.

  diskutil cs list