It really depends on the agency (and the book). Some will have a longer process, others will move more quickly. If they're smaller, they might just have the few agents in the agency read every full request (my agent does this). If they're larger, they might have the boss read everything that comes through, or they might share with colleagues they trust. They might check the market to look for other books like it to determine sales potential. They might Google the author to find out more info. They might grab the phone and call as soon as the last page hits the floor if they love it that much and think it'll go fast.
There's really no set way any of them work. Even if they love it, some will call and others will email. I got both.
Very generally though, they read it, they like it, and they'll act on it if they want to rep it. If it's a hot property (meaning there's something about it that just screams quick seller) then they'll likely move faster. If it's just a great book, then they might move slower.
