In the document tab of a transaction, you can view all the document shares you have sent out for this transaction.
Click the tab called Signature Requests. These requests do not expire until the agent cancels them.
It will load all the signature requests you have out for this transaction.
You can resend the request by clicking the Resend button. This will bring up a window to resend the signature request, which you can see in the solution Documents: Resending Signature Requests.