When is historical fiction historical enough?

We've been thinking about historical fiction characters lately.  Specifically, we've been wondering at what point a historical fiction subject is, well, historical enough for the novel to work well.

For example, we'd be willing to bet that if a historical novel were published tomorrow with a central protagonist of....

....that it would flop.

And yet if the 6,842,971st novel was published about another famous royal:

...well, we're pretty sure that novel would sell just dandy.

So what's the difference between these two scandalous, royal women?  Both are royal.  Both are dead.  Both are historical. 

Is there a cut-off date for historical fiction?  It seems to us there is, although we couldn't tell you what that nebulous date would be.  Perhaps the character shouldn't be anyone that exists as a real person in the reader's memory.  Perhaps a character needs to have lived so far in the distant past that they don't seem quite real to us, allowing the author to create their world for us.

What's your magical "cut-off" date?