Why is my time machine restore taking so long?

No it is not normal. You may have Bad Blocks on your backup drive.

An alternate way to get there is to Install a fresh copy of MacOS from fundamental sources, then add your data using Setup assistant on first run.

I assume that it has completed its restore for you by now.

Data restoration via T.M. will take different amounts of time depending on the Mac computer: the amount of data that is being backed up (everything covered by your usual time machine backup), the speed of your connection to your Time Machine drive, and the speed of your Mac's internal drive. If your Time Machine drive is connected by USB then its limited by your USB speed (USB3 or USB2). If it is on a network (Time Capsule) then connecting by Ethernet cable is a much better option instead of WiFi).

The recovery mode process will take a long time to read all the data which it will try to restore, and this can make predictions about duration which are wildly inaccurate. My last restore predicted about 24 hours (for about 300GB to restore to a standard HD), and I was surprised when it later changed its mind and predicted completion in a couple of hours.

if you get no prediction at all within an hour or so, I would guess that something else is wrong. Try verifying the Time Machine backup from another computer (alt-click the time machine icon in the menu bar). In case someone else can be more help, please post details of your iMac, what sort of disk your Time Machine backup is on (USB2/USB3/FireWire, HDD, SSD) and how you connect to it (direct cable, network connection, WiFi), size of backup etc.

Also, take the help of this article to know the key reasons responsible for long time while data restoring via Time Machine.