I agree with the idea that it depends on the context. and presumably, at no time in public is there areola or "nippleage" involved.
I think a dollar bill can be used in deciding how much cleavage is right.
Church: None. cleavage is covered, nowhere to place the bill.
Kids school: dollar folded lengthwise in half, would stick vertically in the displayed cleavage.
Out on errrants, on a walk at the park, etc.: Unfolded dollar bill fits vertically in cleavage.
At a bar, out dancing, at a play, etc: Go for broke: dollar bill fits horizontally in cleavage (presuming you got room).
Ok, I had too much fun figuring out the dollar bill system.
After I wrote this out I found the perfect illustration of too much cleavage: http://www.newsnet5.com/news/19886368/detail.html
WARREN, Ohio -- Police in Warren, Ohio, released video of a woman who was interrogated by officers with a baby squirrel in her shirt.
She was brought into the station to be questioned about a crime.
The detective sat amazed as the woman continued the conversation while tucking the squirrel into her cleavage.
Police let the woman and her animal friend go.
Go to the link and watch