Hi I have a iMac which I’m reinstalling macOS it’s been over a hour and display it say it still has 8 hours left is this normal

A reinstall won’t help performance. The reason your iMac is very slow is due to the 5400 rpm hard drive. Very slow. If you use an external SSD and run macOS from that, it will make your Mac 10 to 30 times faster for storage. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

Also, please make a Time Machine backup!

There is NO issue with the hard drive’s condition or health, you just bought an iMac model with a slow hard drive. It’s a limitation.

Welcome to the Apple Support Communities.

Please respond to all these questions:

• Why are you reinstalling macOS?
• Have you erased your hard drive?
• What storage media does your iMac have (Hard drive, SSD, Fusion Drive)?
• What is your internet speed?
• Are you installing over Wi-Fi or Ethernet?

Cheers,

Jack

Have you erased your hard drive yet?

Excellent. Here is what to do:

Go to the Apple menu (top left) > Startup Disk, and select your startup drive. Click "Restart".

Once you are back into macOS, to determine why your Mac is slow, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack

Also, do you have access to another Mac and what year is your iMac?

Sorry for all the questions, just trying to proceed with the best plan, but have you erased your hard drive yet? (If you have not, do not!)