It is going to depend on each individual image and the way a user perceives it. I do not believe there is any magical number that works for all.
You're killin' my dreams buddy
So, are there images you shouldn't bother using this tool on? How do you know when going through this step isn't going to result in an appreciable difference?
Should I basically just use it like sharpening? View at 100%, start at 0, go up until I see problems, then back it down? Then recheck at normal viewing size?
Remember, your CR2 files grow to very large size, 3X their normal size, so its not something to use on the 800 images you shot last night.
For images that need DLO, I apply DLO in RAW, convert to JPEG, then I un-apply DLO before closing the RAW, so the RAW size does not increase 3x. Undoing DLO is very quick unlike applying it.
DLO doesn't appear to make much difference when viewed on small screens or prints, but viewed large, it really does great for things like chromatic aberration, color fringing, and sharpness, better than the traditional ways of lens correction and sharpening IME in that results look more natural.