The one people seem to be drawing on a lot (I found two recent articles about it) was the raid on Donald Manno in 2008.* The better of the two was here on Reuters.
In 2008, Manno’s law office was raided by federal investigators
pursuing a racketeering case against a suspected mobster who was a
client and friend of the lawyer. The FBI copied Manno’s computer hard
drives and carted away evidence. Three years later, when New Jersey
prosecutors indicted Manno’s client for fraud, money laundering and
extortion, they also charged the lawyer.
While he was ultimately acquitted, Manno spent years defending
himself. He predicted a tough road ahead for Cohen.
It appears the acquittal only happened in 2014. That's 6 years from raid to resolution.
There's a lot more good information in that article about the process.
Only a handful of law firm search warrants have been publicly
disclosed. But based on that record, it is safe to say that law firm
raids do not bode well for lawyers or their clients.
New York defense lawyer Gerald Lefcourt said warrants against lawyers
almost always end in charges, given the high bar the government has to
meet to obtain them. Lefcourt represented defendants in a 1990s
racketeering conspiracy case in which the FBI raided the law firm
Lysaght, Lysaght & Kramer. The law firm’s name partners were
eventually convicted at trial.
There isn't a lot of info I could find online about that LL&K case. There are official court documents though. It appears the named law firm partners after being raided were tried and convicted under RICO for bribing union officials, election law violations, and wire fraud.
So it looks like this is indeed "rare, but not unheard of". The reporting on the kind of activities Cohen was involved in are pretty much exactly what I listed above.
People with knowledge of the inquiry have told The Washington Post
that Cohen is under investigation for possible bank fraud, wire fraud
and campaign finance violations.
* - Most likely this is because he was found not guilty, so he's quite happy to talk about his victory to anyone who will listen. People found guilty don't appear to be quite so ebullient about it.