I've found that the ideal size of Timemachine drive is just about twice of the target drive. It last for months if not years -- enough to go back. Beyond that, there must be an archival/backup system in place if you still want to get back files deleted years ago.

In my experience there's no need to worry about long term disk usage of Time Machine. The backup is going to get in a bad state and start failing to add new backups, requiring you to wipe it and start over before the drive fills up.

My anecdata is the opposite of yours. My backup history is over 15 years old!

I started backing up using Time Machine when I got my first Mac in 2006. Migrated my sparsebundle from my first Time Capsule to another when I needed a bigger one, then migrated that to a number of other drives drives over the years (which thankfully never failed, but I replaced them every 4 years). Have successfully restored from a backup onto at least ten different laptops (I was upgrading every year for a while). I have literally only ever had “one” Time Machine backup in the entire time that I have had a Mac. Honestly, I’m blown away it still works.

I maintain other backups too, I’m not crazy to think that this is rock solid, but at this point I’m keeping it going just to see how long it will last.

That is very impressive!

Up to a few years ago I was constantly hosing my TM drive and starting fresh.

TM has supported multiple destinations for a good long while, so in the past I was always running two TM destinations.

BTW, side note, with SSD prices crashing, I should be able to get rid of the spinning-rust and the luxury of silent local SSD TimeMachines running!

I’ve had this happen enough to just expect it “someday” and learned not to fight it.

When Time Machine goes stupid, just wipe it and start over. Any efforts to try and recover or repair are just folly. Just watch your life tick away for days and days only to inevitably fail.

It doesn’t happen often, but does happen every couple years. Many times it’s an indicator the drive is bad.

That said, I’ve never had a problem recovering from TM. Whether it’s a single file, directory, or a complete reinstall.

I have TM for my main drive, and use BackBlaze for everything (including my main and media drives).

In my experience, I've been using TM ever since it was released, without these problems - and I've used it to transfer to new machines multiple times.

I've also never had a problem with Time Machine since it launched.

I used to find that.

Lately, for the last few years, I haven't had that problem.

Before TrueNAS I was using an M1 Mac Mini with a USB3 JBOD as TM destinations.