By how much will daylight change for partial-eclipse observers?

Of course if you are on the path of totality, you will lose nearly 100% of your normal daylight illumination when totality happens. It will seem like late-twilight with a narrow ring of daylight encircling your horizon. If you are only able to watch the partial eclipse, your experience will be quite different. Unless the sun is at least 75% covered by the moon, you may not even notice much illumination change at all unless you are aware the eclipse is happening.  It is only by the time the eclipse reaches about 90% coverage that you will start to notice a landscape dimming, and by about 95% you will definitely be aware of something happening to the sunlight even if you did not know there was an eclipse at that moment. The details, of course, will change depending on how aware you are of the exact timing of the event, and even your emotional state and its impact on your sensory acuity. One thing you may notice once the eclipse reaches about 90% is that the shadows on the ground will start to seem less sharp as the light source (the illuminated sun) becomes smaller.  Also look for crescent-shaped shadows cast by light passing through gaps in the leaves on trees.