What is "normal" System Data size for an M1 iMac on macOS Sonoma?

There is no "normal" size for System Data because System Data doesn't exist.

Yes. You heard that right. It doesn't exist. Or perhaps one might say that it is mis-named. The word "system data" suggests that it is part of the operating system. It is not. Or rather, it's something else.

This entry appears in a list of categories of file types. In fact, virtually all of these entries are wrong. When you see things like "Applications" or "Documents", you probably think they relate to your Applications folder or Documents folder. They don't. Make sense yet? I hope not.

These are categorizations of file types, not actual files or folders. In any list of categorizations, there must be an "other" type. This "system data" is actually simply "other".

You may have found some overlaps between hidden files, caches, and mail attachments. But you can certainly have hidden files, cache files, and mail attachments that aren't "system data" (i.e. "other").

The more important question is what you should do about it. The answer is "nothing".

The only thing you need to worry about is your free space. Again, names are important here. The operating system will tell you all about "available" storage. "available" is not the same as "free". In fact, "available" is simply another name for "used". Make sense? Again, I sure hope not.

In Disk Utility, when clicking on a volume, you can see a value for "Free". This is the only actual "free" storage value that you can rely on (for the most part). This is the only value that matters. Make sure it is at least 100 GB to 200 GB in size. Anything less and you'll start to have random problems. You'll try to fix them and the problems will only get worse. Then you'll leave in frustration only to find the problems mysterious fix themselves on their own. After a while, the whole process starts again. But if you keep at least 100 GB to 200 GB of actual, truly "free" storage, then you won't have those problems at all.

There is no "normal" size. At least, Apple, AFAIK, has not published anything related to what size is normal.

System Data is controlled by macOS. One of the largest contributors to its size is Time Machine local snapshots. There are other contributors as well. System Data is dynamic. That is, its size can and will change constantly over the course of time as macOS manages it.

The dynamic and allows changing values referred to earlier are replaced to Purgeable Space

It is generally a good computer practice to alway keep at least 20% to 25% of the Total Drive Capacity’s as Empty Space.

Allowing the computer to drop below these guidelines may eventually, cause unintended consequences.

There is Purgeable Space and there is Empty Space.

Purgeable Space which is Controlled by the Operating System.

When the Operating Systems decides the computer needs additional Empty Space, it will move a portion of the Purgeable to Empty space

AFAIK - there is no User Actions to hasten this transition from Purgeable to Empty Space

It can day or longer before this will occur.