Some quick background:
I began querying for a new agent for my novel in January, 2006. (My orginal agent and I parted ways on excellent terms the previous fall. I still email him occasionally.)
I sent letters and emails to agencies, depending on their preferences as indicated on their web sites or in various trade listings. One agent holds the record for the fastest response. I emailed him one evening and had a reply in about 15 minutes. I thanked him for his speed, commenting that he was working late (it was after 10 pm in New York). He replied again, saying this was his normal routine.
Too bad he didn't want to sign up my project! I like his work ethic.
On the other end of the spectrum, I still have about 8-10 queries out there for which I've received no response at all. I'm not including emails, because often the agency will tell you that "no response = no interest." No, I'm talking about letters sent via snail mail. WITH a stamped, self-addressed envelope for a reply.
Miss Snark says that almost all agencies, no matter how bad they think your writing is, or how busy they are, will eventually return those SASEs with some kind of response. Before today, the last one I received was back in September, five months after I'd mailed my query. That length of delay tickled me enough that I sent them a quick email thanking them for replying and telling them that I laughed when I saw the envelope in the mail.
Well, today another reply came in. This one from an agency I'd queried on March 23, 2006. It took them 11 full months to send me my own stamped envelope back!
That's the new record (for me). But I've heard of even longer delays in responding.
One more tidbit about this response (and notice I'm carefully NOT identifying the agent!) . . .
The returned SASE was empty.
BUT, handwritten on the back was, "Thanks, but not for us. (Name of agency). 02/20/07"
Hey, I consider myself fortunate. I got an actual handwritten reply. Usually it's just an unsigned, undated form letter or little card stuck in the envelope.
And they even sealed the envelope. Wow!