I like writing full biographies because it helps me with my issue with length. It also forces me to put more effort in the rest of the profile, so it's up to par with the biography, which is one of my strengths.
I think too detailed is when your biography is more than two pages. One and a half is usually a good aim. Sometimes its best to write down important ages, or look at a chart and see when a childs development most changes, and make a list of events that affect your character as who she is today. I usually like to give a brief description of the parents, how that affected the home life, and a few details on how the character grew up.
To me an 'acceptable' detailed fact could be something along the lines of...
'She was the product of a revenge affair between the two parents', or 'her childhood best friend was like this (and go and list the traits of a friend and perhaps why they got along so well)'.
However, I don't think there is a 'right' way to do it. Biographies shouldn't' be read as a list, but as a sort of mini-story that has more fact than narration. It really depends on yours and the moderators personal preferences.