It's shockingly high. Most western countries see an average of 26% - 40% ad block rates (by pageview) across the web. Some sites skewing to an older, less tech savvy audience, see between 8% and 15%, while tech heavy sites can go as high as 92% of pageviews being ad blocked. It's actually crazy that adblock rates in developing countries like India and Indonesia can actually be much higher on average because of UC Browser adoption and high mobile web consumption. They don't care about ads and privacy though -- just saving money on their data plans from not downloading ads.

Source: I was briefly a product lead for a large tech company's anti-adblocking efforts and worked with publishers and others globally to measure these rates.