Check your clock. It's been five minutes since you sent it. Time to refresh the inbox.
The first 24 hours are the worst. You are sure you will never make it the time period listed on the agent's website (you checked that before you submitted, right?), but to general surprise, you make it.
Then, the unthinkable happens. You get a response. Yay? You're not sure if you should rejoice or not. With trepidation, you click on the open button.
Cue the trumpets. Mr. McAgent is requesting the full! Jump for joy! Cry out to the heavens in exaltation! Then, sit down quickly, because you remember you're checking your email using the free WiFi in Barnes and Noble, and people are looking at you oddly.
Once you calm down and are sure they aren't going to throw you out, it's time to think. What now?
First, the manuscript. You've formatted it into proper manuscript format, haven't you?
What, you didn't?! Why'd you send out a query without first making sure you have your book in manuscript format?
Ok, you really did have it in manuscript format. Sigh with relief. What's next?
Second, you calm down. This person is trying to come work for you. Remember this before you begin, because although you are so excited, you want to be sure to act with professionalism. Be grateful, but not bubbling. Also, not to pour cold water over your happy dance, but sometimes agents only request fulls. As well, sometimes agents will request fulls for anything that sounds interesting. It doesn't mean that they've begun writing your name on the contract. Still, you have a foot in the door. Now's the time to shine.
Next, check the email from old Johnny boy. Did he ask for exclusivity? He did? Ok, now here's what you do.
Begin your reply to Mr. McAgent. Be courteous and gracious. Thank him for his time. Then, tell him that he has the novel for an exclusivity period of _______ weeks (the email may say a time period, or if not, then try to suggest something that won't be insulting or rushing, but is something you can live with and isn't 6 months either). Don't forget to mention that if they need longer, that is obviously perfectly ok, and that they should reply back letting you know how much time they need.
You're not setting a deadline for the agent. You aren't saying, "You must have this back to me by this time." What you are doing is signifying that the agent does not have the novel indefinitely. Do not let an agent hold your novel hostage by not setting up an exclusivity period. What can happen is that the agent gets bogged down and doesn't get back to you for months on end, and you can't submit to anyone else in the mean time (the idea behind exclusivity). Exclusivity is fine and is a good thing as long as you have a period in place.
Just don't get too caught up in that part of it. If the agent replies back later on needing more time (possible, perhaps even probable), then give it to them. Their desire to spend more time considering is often a good thing, if you know they are considering.
It is also a good idea to send a reminder email if the agent is getting close to the end of the exclusivity period, just reminding that exclusivity expires on X date (a few days after the email), and saying that it's fine if they need more time. Thus, the exclusivity has allowed you a reason to retain contact with the agent.
Other than the above email, DO NOT BUG THE AGENT! They are busy. If you send them an email every couple days, they won't thank you for it. Be professional, and above all else, be patient.
It's hard. I know.