How Long to Wait Before Following Up?

Agent A requested a full about 7 weeks ago. I snailed it to her, so let's say she's had it 6 weeks ago. At that time, agents B, C, and D also had a full or partial. In the meantime, Agent E has asked for a full, but wants it exclusively. Now, I've heard back from B, C, and D, but A is still out. I don't want to send it to anyone else, because I want to let E have it exclusively, if that's what she requires.

So--when should I ask Agent A if she's still interested? (Or whatever you say when you follow up?)

Thanks!

Thanks, DeadlyAccurate. I have already told E that someone else still has it and that I'd send it her way if they are not interested. She thanked me for my honesty and said to send it when they'd finished. I suppose I (foolishly) thought that because Agent A responded within 11 minutes of me sending my e-query, that she'd be waiting for the postman at the front desk, charge back to her office, read the (brilliant) MS in one sitting, scream "eureka! Here is someone who will knock that pesky JK Rowling off her perch!" and get straight on the phone to me. (After that scenario, does anyone here think I should write fantasy rather than historical fiction? )