Janet, do all agents read a query on the first day? And is the tagline that they'll get back to authors in 3-4 months just a line?
No they don't. I've been told I'm one of the fastest readers, and as you can see from the blogroll on the right I'm only caught up through 7/14 as of today.
I know several VERY good agents who take a week or more to read queries. And lots more who take a LOT longer.
BUT the industry standard on query response is not 3-4 months. It's 30 days. And given my colleagues loathsome practice of no response means no (insert rant here), 30 days is about the outside limit to wait for a reply.
HOWEVER you should BE READY in case the agent reads your query the exact minute you send it. I've had this happen several times. Some writers are certain that a quick reply means I didn't read the query; others aren't ready to send the manuscript when I've requested it. If you're making a list of best practices: BE READY to send what an agent wants BEFORE you send the query is a good one to list.