Waiting, waiting, waiting...

My novel Shaketown has been at an agent's for over six weeks now. This is the second go-round for the book; initially, it was read by two agency readers who "loved it", and passed it on to the major agent. She went through the book and made some good suggestions. I spoke to her on the phone, liked her very much, and followed through; I returned the book to her since she had spent so much time on it the first go-round--I felt she should have the first shot at selling it. I'm getting frustrated. I know Shaketown is a darn good historical yarn with interesting characters (who doesn't want to read about a feisty madam and a clever Chinese scholar/poet in Victorian San Francisco?) and a twisty story line. What to do next? Start querying other agents (I have a selected list) or wait a little longer?
For now, I wait....