This Free App Shows How Long You've Been Using Your Mac
Pandan will tell you how long you've been using your computer—without bogging you down in metrics.

I've tried all kinds of time-tracking apps over the years and they can be helpful, but it's easy to get bogged down in the numbers. Sometimes all you need is a little self-awareness about how long you've been on the computer, either all day or during the current session, without the burden of long-term metrics.
Pandan is a totally free Mac app from indie app developer Sindre Sorhus that that does exactly that. The application mostly lives in the menu bar, telling you how long your current computing session has been. You can click the menu bar icon to see how long the previous session was and how long you've been on your computer today.

You can also dig into the history, if you want—the applications goes back 180 days. But history isn't the point of this application, which is very much not a time-tracking app—Sorhus recommends looking into Screen Time if you want to track longterm trends. Pandan is instead about gently reminding you how long you've been on the computer so you can be aware of that.
The settings let you set up an optional reminder when you've been on your computer for a particular amount of time. These can take the form of a macOS notification, a pop-up near the menu bar icon, or a HUD in the middle of the screen as seen at the top of this article. The idea is to remind yourself to get up or do some stretches.
You can even trigger an Apple Shortcut if you want, which means you can set up your computer to do all sorts of things when it's time for a break. I can't exactly explain everything Apple Shortcuts can do—it's a powerful tool—but know that this means you could do things like load particular applications or play a custom sound when you've been on your computer for a certain amount of time. And Pandan can also be triggered using Apple Shortcuts, meaning you could use it as part of productivity automations you've already set up.
That's all power user stuff, though. For most people this application doesn't need to do anything other than the basics, and that's telling you—at a glance—how long you've been at your computer. It's very good at that, and completely free, all from a developer with a great reputation. Check it out if you're interested.