Applaud I spend a lot of time glassing and I would agree with the ranges Robby posted. Overall I think it really depends on the terrain and atmospheric conditions even more than the distance. I’ve picked up deer at 2 miles with 8x…and also picked up deer at 500 yards with 15s that I missed with 8x (could have been moving too fast also!).
I start reaching for 15s beyond 1000 yards or in the 500-1000 yard range if the brush is thick or early/late lighting conditions when they can blend in better. If I know the spot I’m hunting is all going to be within 1000 yards or so, I might not even bring the 15s.
BTX (STC big eyes for me) really shine at a mile and beyond. There is overlap with the 15s at a mile or maybe two in good lighting conditions. But many times I’ve done a pass at 15x or 18x and come back through at 25 or 30x, and spotted deer that I missed.
So much depends on the terrain (tall sagebrush/bushes and mid season gray coats make me want more magnification…early season red coats and green grass make me want to pack less magnification). It also really depends on your brain/eyes. It’s pretty easy to glass right over a deer, switch to higher power, and then BAM you see him and wonder how you just missed it. Someone elses eyes/brain may have picked it up on the lower power. Everyone is a little different I think in this regard and it takes some experience to figure out what works for you..and mentally being more/less focused or tired plays a huge role as well.
While I’m glassing my mind is constantly working on the subconscious level. Is there thick/tall enough brush that I could easily miss a deer in this stuff? Is the lighting bad making it hard to resolve details? Is there broken enough country that a deer could take one step and disappear or reappear? All of these things go into decisions to make another pass, focus longer on specific pieces of terrain, switch to higher power for the next pass, or pick up and move.