In the document tab of a transaction you have the option to view all the document shares you have sent out for this transaction. Scroll down to the section called Signature Requests. These request do not expire until the agent cancels the request.
After clicking on "Click here to show signature requests." it will load all the signature requests you have out for this transaction.
You can resend the request by clicking the Resend button and it will bring up a window to resend the signature request which you can see the the solution Documents: Resending Signature Requests.