How long will it take an agent to get back on a query? For some, never. Aaron Priest and Lisa Vance do not respond to queries if they are not interested, and it does not matter if you included a SASE or not.
Amy Rennert took four months to reject my snail-mail query, and Carolyn Grayson needed just over three months to stuff a form rejection in the envelope. Some agents are much faster, like Julie Barer, who opened the query, took one look at it, and immediately shoved a rejection letter in the SASE. Turn around time: 2 weeks.
My experiences with InkWell Management vary, depending on the agent queried. Alexis Hurley responded to the e-query after two weeks, and Catherine Drayton said no within three days.
Over at The Creative Culture, they don't respond to e-queries if they are not interested. Needless to say, I have never had a response from any of their agents.
The big names tend to respond faster, it seems, but that may be due to the fact that they have staff to do the heavy lifting. At Sterling Lord Literistic, Claudia Cross rejected a query in a matter of days, or possibly minutes. On the other hand, Robert Guinsler had a full manuscript for a year before he finally turned it down.
There is something to be said for snail mail queries - at least you get a response eventually, most of the time. Better than staring at the sent mail box, wondering if the agent ever got it, if it was caught in a spam filter, if it was merely deleted on purpose or by mistake...you could go on for days. So instead of waiting for the agent to get back to you, go write something. Keeps those idle hands busy.