It also depends on the agent. Some are strictly business, and will only contact you when there's something important to pass along (ie, an offer on your book). Others are a bit chattier. My agent and I have talked about books in the genre, good pizza, and the unbearable cuteness that is my kids, all interspersed with his efforts to sell my books.
Miss Snark talked about this a while back, though I don't have the exact link. I believe she said that even for the strictly business crowd, it's not unreasonable for the author to check in from time to time (maybe once/month?) to see how things are going.
Overall, when we're working on an offer from a publisher, I'm sometimes talking to my agent on the phone or by e-mail almost every day. When nothing much is happening, it's much closer to the once/month check-in.