I know this won't apply to all or even maybe most of what you named here. As you may know I've been on a fitness kick lately and have been scaling up a fairly challenging and time-consuming routine. I do most exercises all on one day, the next is my rest day. It's very simple and I just keep that pattern going.

The core exercises I have been doing are push-ups, jumping rope, some lunges, and run/walking. Sometimes I throw other random things on top for fun.

Especially with the easiest quantifiable things (push-ups - count; run/walking when on a measured & marked track) my goal every exercise day is to simply do a little more, do a little better, than I did last time. If I could do 10 push-ups two days ago I figure I can manage 12 today, and so on. One day I ran 3x .75 miles (with breaks in between). The next day I ran I ran 2x 1.1 miles (with a break but shorter).

I know that with all things there will be set-backs, plateaus, bad days, sick days, injuries, etc, and I also can't expect to continually do more push-ups each time, or always run further than I did last time. But for now in this phase, I know I am done an exercise or work-out portion for the day, when I have done more than I did last time. I don't allow myself to stop/take a break/"go easy" if I haven't achieved at least what I did last time. If I could do it last time without breaks, and I had a full rest day to recover, I can do it again.

There are other angles to work improvement, too. Like upping the # of push-ups I do in a set, running faster, etc. I don't have a "done" or an "end goal" really. I want to be healthy, happy, busy, fit. And fitness is one of those forever projects, so hopefully, I'll never be done with it (as long as I'm alive).

But I did want to chime in that the way I answer that question on a very literal day to day level is "When I have done more than I did before/when I have pushed myself harder than last time/when I have challenged myself (and hopefully won)."