when did Harry become unfrozen?

[wynnleaf]

No: HPFGUIDX 160768

I cannot remember where we learned (at least I thought we learned), 
that a spell such as the one Dumbledore used to freeze Harry on the 
tower, would automatically end with the death of the person that 
cast it.

The reason I'm wondering is because at the moment that Snape AK's 
Dumbledore, Harry -- being shocked -- tries to scream, but it is a 
silent and unmoving scream.  If the AK had killed Dumbledore, 
wouldn't Harry's scream have become audible the moment Dumbledore 
died?

Then, of course, all of the Death Eaters and Draco exit the tower in 
turn before Harry becomes unfrozen and follows.  

We can imagine that Harry had actually become unfrozen prior to that 
and only realized it after everyone left the tower.  But the silent 
scream seems to contradict that.  The scream would have been 
involuntary, and one would expect that it would have become audible 
the moment Harry was released from Dumbledore's spell.

So the question I'm wondering is what is the evidence for a spell 
ending the moment the person who cast it dies?  I feel sure we got 
this info somewhere either in the books or in JKR interviews, but 
can't find it.

wynnleaf, who would happily receive an email with this info if you 
don't want to spend a post on it.