While email can theoretically be kept forever, most companies have defined retention programs that are put in place specifically to limit the scope of e-discovery in the event of litigation. There are documents that must be kept per various industries, regulations,etc., but generally, everything else is deleted in a period defined by a given companies retention policy. Most retention periods are pretty short in my experience.
I don't know the specifics of the retention requirements for IRS email, but email can be setup to make them disappear pretty quickly. I'm sure the bureaucrats would rather get busted for incompetent document retention than collusion to target political opponents.
I would presume it would depend on the field. But, as HDD space has dropped in $$ like a rock, AND adding that Lerner wasn’t just some low-level employee, most IT/co (let alone the employee themselves) keep as much...for as long as possible.