I think the average Bear market length since 1929 is actually only 15 months.
Sort of makes sense. Fear, panic, dread - all happens pretty quickly as everyone heads for the exits.
Average decline I'm pretty sure is around 30-35%, so the GFC was about 15-20% worse than normal. You have to wonder if that will be a new normal with all the leverage via artificial instruments today. Perhaps that's somewhat counterbalanced by the arsenal of creative methods they find to pump-prime the economy these days.