Expiration of invoices & proposals are set according to the payment terms of the document (set in edit mode at the very top of the document) - this will help encourage more timely handling of the documents you send your clients. For example, 'net 30' means the document link will expire 30 days after the file was sent. 'Net 60' means the document link will expire 60 days after file was sent, etc. If 'due on receipt' or 'custom' payment terms are set then expiration defaults to 15 days from the sent date. 

Only the emailed link to the document expires, the document itself does not expire. Your client can always access expired documents, open files, and closed files from their project dashboard. Learn more about sharing the project dashboard with your client here.