Acquiring a literary agent is one of the most exciting moments in a writer's life. With it comes validation and a sense that your work is going to go somewhere. 'I'd like to offer you representation,' are probably the sweetest words an aspiring author ever hears. However, the relationship between author and agent is like any relationship, it takes work on both ends. So what happens after the glow wears off, time has passed, and you feel like you're the only one doing the work? How do you know when it's time to move on? I hope you never have to answer either of those questions, but chances are if your career is a long one, you'll have to at some point.
Hopefully you've been attending conferences, retreats, writer's groups, or at least joining on line writing forums and have met other writers. Getting a published author's opinion of what is normal and expected in an author/agent relationship is worth it's weight in gold. Knowing when to push and when not to is a careful balance you must maintain. No one likes a client who calls or e-mails every week to see what's up and ask if there is any news. On the other end of the scale, if you only e-mail once a month or less, your agent will most likely be happy to speak to you. The first thing you have to understand though is that if there's any news, they'll get a hold of you. That doesn't mean you can't ask questions or update them on how your writing is going. But remember, it's a business relationship. For the most part, your correspondence should remain business like. Chances are you're as yet unpublished client, which means you haven't made them any money yet, so don't be a pest.
But how do you know when it's time to move on? If your only e-mailing once in a while with legitimate questions or concerns and not getting any response, chances are your agent is either too busy with paying clients, or is losing interest in your manuscript. If you think the latter may be the case, talk to them, don't be afraid to ask. When an agent loses their excitement for your work it may be time to move on. Chances are they'll be honest with you about this. Thank them, hang onto your dignity, take your manuscript and move on.
Here's a great link to an article on '5 signs a Literary Agent is a Good Match For You':