How much sagging is considered normal for floors in old (100+ years) houses?
Assuming the sag is new/progressing.  

There are countless hundred+ year old homes with proper structure that have floors that don't sag, bounce or shake.

Sagging is never good.  Sagging is a sign of either poor engineering to begin with or structural damage done later.  

bcworkz does raise a good point about older foundation methods and short term sags that stabilize.  I still maintain that even in such cases, a sag isn't "good", but it may not be deal breaking.