How long is an electron transition ?

[owlal85]

Hi,
An electron transition generates a frequency emission u=E/h . In this case the electron moves from one energy to another.
My question is how long this transition last for each atom ?
If I was observing a single atom transition what would I measure ? A single burst at a frequency u=E/h ?
If yes, how long is this burst ? How many periods is it lasting .
This has never been clear for me .
Any material will be helpfull .
Thanks .

 

[granpa]

when I was growing up the figure that I always read was 10^-8 sec (i.e. forever)