I live in the Alps, and there is a pass nearby which is 18km long and averages over 8% (i.e. elevation gain is about 1500m or 5000 ft over 11 miles). On this climb, there are several sections which average 11% over 1 km. That's about as much as I can do on a loaded touring bike in my lowest gear. There are lots of 20% or steeper climbs around here, but as most are under 1km long, I can do them without getting completely exhausted. The long climbs can be grueling, though.