I'm doing my first backup and my Mac disk is still flashing after 5 hours

It has taken me up to 24 hours to do a first backup on Time Machine. After that it backs up every hour, and that backup is usually very quick.

Depends on how large your internal drive is that requires backing up and how fast your backup drive is.

For time machine, the bigger your drive, the more weeks or months it will backup. To get max benefit I suggest buying the largest size disk yo van afford. Do the same for your photos. For time machine, a fast drive is not necessary. The first backup may take a long time, but after that it will be working seamlessly in the background. For photos use the fastest solid-state external drive that you can afford. When comparing, what to buy, check read and write speeds, and capacity. Use Thunderbolt 4 cables, not older ones. You can tell if is Thunderbolt 4 as the end of the cable has a lightening symbol and a number 4 on it.

Thanks for the good information in the thread. Are there any benchmarks on the time for the first Time Machine backup? I realize there are many variables. My initial backup is three days in and has copied about 140GB of about 1.8TB (based on previous estimates shown in TM. The current backup can't seem to estimate remaining time.). If my arithmetic is right I'm looking at 37.5 days total for the initial backup. My faulty memory says this used to take about a week.

Yes.