Multi-threat situation [fen "7k/8/4n3/2b5/3p4/8/2N1N3/3Q3K w - - 0 1"]


This picture above depicts a very simplified situation when white pieces threaten and black pieces defend the black pawn. If the next move is to white, it is a series of captures that is coming.

How does one decide if that is good for black or not?
Is there an algorithm for that?
How long will this series of exchanges take? Until all black and white pieces are consumed, or is there a point when the exchanges should be stopped?
You appear to be referring ("series of hits") to an instructional concept in chess tactics called Counting.

NM Dan Heisman's famous Novice Nook articles includes a multi-part series dedicated to Counting which you can read (preferably in order) at: 


Counting Primer: http://www.chesscafe.com/text/heisman15.pdf
The Most Important Tactic: http://www.chesscafe.com/text/heisman35.pdf
Is it Safe: http://www.chesscafe.com/text/heisman65.pdf
Two types of Counting Problems: http://www.chesscafe.com/text/heisman75.pdf
The Safety Table: http://www.chesscafe.com/text/heisman92.pdf


Article #2 (Most Important Tactic) even contains positions similar to what you posted above.

When do you stop counting? If you were to expand on counting and instead ask "when do you stop calculating forcing lines?" I'd refer you to another one of Dan's articles regarding Quiescence Errors.