Comment by inigyou

Various ISPs give out either a /64, a /56 or a /48. Anything else is very unusual. A normal approach is to limit by /64 at first, limit a /56 to 3-4 times the rate limit of a /64 and a /48 to 3-4 times that again. Someone who has a /48 gets to enjoy 16 times the rate limit of someone who has a /64 but that's not too bad, and you don't have to tune anything per ISP.

Anyone who has bigger than /48 is no longer an individual user. They could be an ISP set up solely for scraping, though that comes with some fees and requirements.