I would not look for a sharp drop too quickly when the rally ends. Likely quite a bit of churning at high levels before a really bad break.
I still think the real reason for the strenght of this move is market conviction that the Fed will keep interest rates -- both long and short -- at current extraordinarily low levels for some time no matter how high stocks go.
This is the old Greenspan put multiplied exponentially.
“The things that will destroy us are: politics without principle; pleasure without conscience; wealth without work; knowledge without character; business without morality; science without humanity; and worship without sacrifice.” Mahatma Gandhi