Gift Cards expire one year from the date of issue. The date assigned/issued and the expiration date are listed in the "Gift Cards Available" section of your Back Office.
You can send the same code to as many people as you like within that year, until one of them redeems the code. It's entirely up to you how long you give someone to redeem a code. If you give them too long, you run the risk of losing the code since they do have an expiration date.
For Example: If you have Gift Code 123456789 available in your back office and share that code with your cousin and they do not use the code within a reasonable amount of time, you can resend that code to your friend who is wanting to purchase today if it is still available. Once your friend has used the code, if your cousin tries to use it it would not work. You could, however, send your cousin another code when they are ready to make a purchase.