Don’t be surprised if the querying process proves one of the darkest times in your life as a writer (as is the case for many of us). Querying is like going on blind dates as a really unattractive person. It is asynchronous, internet blind dating that involves such long waiting times you’re often not sure if you’ve been stood up entirely (you can feel free to assume so if you haven’t received a response after a year).
Your query (with or without the first 5 pages) is the first, blind date–you will make a good impression, or get a “not my type” step-aside.
A partial request for the first 3 chapters (or up to 50 pages) is the second date–getting to know your book better, considering getting serious with you.
A full request for the entire manuscript is the third date, when the agent sees everything–and decides if you are well-matched enough that you can sign a contract and make it official.
Try not to query more than 6-8 agents at a time. No bites, tweak your query. No bites after partials, fix your pages. No bites after fulls, work on the whole manuscript (hopefully they gave constructive advice).
You can record your queries on Query Tracker.