I sent out my first query for my first completed novel in August of 2005. Since then, I've written 3 novels and most of a 4th.
According to my agent tracker database:
Total queries: 99 (for 3 completed projects)
Date spanned: 8/4/05 through 11/9/07
No answer post 1 year: 22
Outstanding queries: 6
Requested fulls awaiting response: 2
What I find fascinating about this is the high percentage of non-responses. Greater than 1/5. And this is after scrupulously following agency guidelines in terms of email vs snail mail, SASE, sample pages, etc. Some, but not most of these agencies do indicate that no response means not interested, but the vast majority of the non-responders have language on their websites or other postings on time frames. Several agencies did not respond even to a polite status request.
I share these numbers only as a point of interest. For me, this reinforces the need to query widely. To broadcast, as in its original meaning: "Sow over a wide area, especially by hand."
Many, many of your seeds will be pecked by birds*, get too much or not enough water, or will be buried too far down in the soil to reach the sunlight.
Cast your seeds widely, looking for fertile ground and good growing conditions.
*A joke for my hubby, who is a physician. There is an ICD-9 (diagnostic) code for just about every illness or ailment, including one for 'pecked by bird' E906.8 For some strange reason, this one makes me laugh, though I realize if one were pecked by birds, it would not be very funny. What can I say--I have an odd and often macabre sense of humor.