Offer timeline question

[Stephen King]

So I'm a little embarrassed about this, but....

I put in an offer on what would be my first investment property -- was so excited! Wrote the earnest money check, signed all the paperwork at my buyer's agent's office. Like most newbies I'm starting without a lot of cash, so I gave the seller some of what he wanted in exchange for some of what I wanted, figuring it was a good place to start discussions. Or end them. One way or another, though, at least I was starting to start. 

That was three weeks ago. I've queried my agent several times and keep getting a "he hasn't responded" response. I vividly recall the part of the mentorship I went through where he discussed how to set up the timeline on offers and contracts, with an expiration date on the offer and so forth -- aaannnnd, I just forgot about it in the excitement of writing. And I checked, and there's no place for it on the canned offer letter that the agent used.

So. What are my options? Is there any sort of natural expiration date for an offer letter? Or am I stuck waiting on a response to know whether the earnest money check will be cashed or returned? 

Thanks!