I have so little power (250 watts, 40nm w/ 418 Wh battery), and so few levels of assist (3), that the choices are pretty clear for the two types of riding that I do, and I use all three levels for both types of riding.
For general fitness rides when I'm not training aggressively, I'm shifting levels constantly, but I use "HIGH" for anything but gentle hills, "NORM" for gentle hills, and a combination of "ECO" and "NORM" for rolling terrain. For flat segments, I shut off assist completely.
For distance rides, which are up to 35 miles so far (and longer this fall, I hope) the only difference is that I use "NORM" for short, steep hills whenever I can, and for longer steep hills, I'll hang on to "NORM" for longer before shifting to "HIGH." I don't worry too much about "ECO" vs. "OFF" because it has less impact on range, but on gentle terrain, I probably hang on to "OFF" for longer before shifting to "ECO" for this kind of riding.
Given how little power my motor has, I don't worry about wear and tear on the chain, and so far, this hasn't been an issue. Believe it or not, I've only ridden about 1,000 miles in about a year (I have health issues, and the hills here are steep) and both the Park tool and my LBS tell me my chain is doing fine.
@Stefan Mikes -- The battery wear issue is one that I've considered. Just in round numbers, let's say that when I can stay mostly in "NORM," I'm going to get about 42 miles of range, and when I use "HIGH" more, I'll only get about 35 miles of range, so it's clear that I'm shortening the total lifespan of my battery
at least 15% faster when I'm riding with more assist. (In fact, the numbers are more like 45 and 37, I'm just making the math easier!)
However, I think the real number is higher, maybe more like 20%, though it's only a guess. Sustained periods of high assist just intuitively to stress the battery so much more based on how quickly the battery bars get used up.
For my most brutal ascents, I'm in HIGH and granny gear for most of the time-- but I still will knock it down to "NORM" or even "ECO"
every time the grade gets a little flatter, even if it's only for 25 to 50 feet, just to decrease stress on the battery (mostly) and the motor (somewhat.) I also stop frequently-- like, at least six times when I'm climbing Brand Park Motorway (3.7 miles, 1,850 feet of vertical, 9% average grade w/ 29% max grade) to reduce stress, and avoid degrading the lifespan of the battery, the motor, and the human.
I don't know if that makes any sense, but whoa... my OCD is really kicking today, huh?