Here's my question: sometimes query replies (in my case rejections so far) come like lightening, and other times it takes several weeks. What I guess I'm wondering is, if it a case of "the longer it's out, the better your chances are" because it's actually being considered, or does the fact that the query is still out just mean it's in a pile somewhere? Is this a total toss-up or is there a "usually" answer?
There are very few hard truths in this industry, and this is definitely an area in which a million things could be going on in the other side. I can tell you this:
(1) The "pile" (referring to the snail mail pile) stacks up based on how often the agent checks it, or if the assistant is in charge of that and if so, how often they come in. Sometimes I only come in once a week and do the whole pile. I used to work three days a week, so the pile didn't build up as much, except over the weekend. We do not go through it in any kind of order based on how much you spent on postage, so don't waste your money. What I do - and it's not always done this way - is go through the whole thing to make sure nothing's a bill or a royalty check or a bank statement, then stack it back up from largest to smallest envelope because it stacks better on the table. Then I do the whole stack.
(2) It's true that we reject instantaneously while we might take more time to think about a maybe query, but this by no means universally true. Generally I make a pile of maybes, and my boss looks at them, picks up the one she likes, and sets them aside. Then when she gets a chance, she emails the person. It's probably within the day because she's polite, but that's not true of everyone. They might let it sit on their shelf for a couple weeks (even a letter) if they are extremely busy. Agenting stuff like contract negotiations, publisher-set submission deadlines of final manuscripts, proof approval, and any kind of conference to get ready for (like the BEA or Frankfurt) are really more demanding and is what the agent does all day. In fact, hope that they do, because it means they're working hard for their clients, and if you become one, you'll want the same treatment.
(3) Partials and fulls can take a long time. This is true. I have had agents get back to me really quickly, generally with a rejection. Or if it's a partial, the agent can take a quick look and say, "Eh... I'm not sure. Need to see the whole thing" and request a full very quickly as well. If you have a full or a partial and it's been 6 months, give them an email. If something comes up, like an offer from that publishing house you sent into a year ago, call them and tell them. They will drop everything and read the manuscript. If they don't, they're definitely not the right agent for you.
(4) Some agents do not feel the need to respond to email queries if it's a rejection. I think this is rude and I'm glad my boss takes the time to reply to her emails, but that's the way some people feel. It's slowly changing as e-queries become more common, but some agents still just ignore it if they don't like it. The paper query with the SASE is a little harder to toss in the trash. There's some guilt factor there.
Beyond that, I can't say. Every agent works differently, and they work differently from week to week depending on what's going on that week. If they're busy badgering an author to get the final manuscript copy into the publisher by the deadline because it's tomorrow, then they won't be reading partials.