The expiration date of the card is printed on the back of the card. This date is composed of month and year, and the card is valid until the last calendar day of that same month. For example, if expiration date is "06/21", the card can be used until midnight June 30, 2021.
Also remember that your card and your Coverflex app will only work as long as you have access to Coverflex. The only exception to this is using the card to pay for food - in which case, and if you have a balance, you have 6 months to use the amount, starting from the day you lost Coverflex access.