Subject: How long does it take for electron to change energy levels?

Hi, thanks in advance for answering my question: I know that when an electron 
is struck by a photon it will move to a higher energy level, but does that 
atom-shell shift take any time to occur? (I've read posts saying it 
is "instantaneous" but I don't believe that anything is truly takes "no time 
at all" to occur if "motion" between Point A and Point B is concerned. There 
are other events that occur, too, for which I'd like to know the time lag 
between cause and effect: For instance, if you fire a "particle" at an atom, 
how much time elapses between the point when the particle "strikes" the 
highest electron shell and the point when an electron is knocked free? I 
believe there must be a time lag in events of this kind because there must be 
a certain "elasticity" within atoms. There is an elasticity within billiard 
balls, and they do not "instantaneously" change direction when they strike one 
another.  

Re: How long does it take for electron to change energy levels?