How long will FileVault encryption take? Can I stop it?

I enabled FileVault to encrypt my hard drive a few days ago. The encryption process is still not finished and the estimated time keeps changing. It's been anything between "4 days" and "300 days". The encryption process running in the background makes my MacBook Air (from mid-2011, OS X Yosemite) heat up very quickly.

I'd quite like to stop the process, or at least know how much longer it will take. How do I do that?

I'm also wondering if, in order to complete the process, I have to keep the computer turned on. Does the computer sleep mode pause the encryption?

I made a back-up of my computer's hard drive before I started the encryption process.

There is no way to stop the initial encryption process once it hat started. But the good news is that you don't need to have your computer running until the process has completed. You can sleep or reboot it anytime you like, the encryption will continue afterwards.

On Macs with a T2 chip FileVault encryption can be enabled instantaneously. The disk is always encrypted and only a new key will be set inside the T2 chip.

See https://support.apple.com/en-us/HT208344