All boards remain accessible online indefinitely. The creator or recipient can choose to delete them at any point, of course. Once a board is deleted, it is permanently removed from our system after 7 days and can not be restored after that time.
The board creator (or a board admin or Org Admin on a Business, Pro, or Enterprise plan) can choose to lock a board at any time as well. Doing so prevents contributors from adding further posts to the board or editing existing posts, and prevents the creator/board admins/Org admins from making any edits to posts, board settings, or delivery settings. Boards can also be unlocked at any time. More on this functionality here: How do I lock/unlock my Kudoboard?
Tip: If you'd like to save a copy of your board to your device, you can always download a JPG of your board. Or, if you'd like a physical copy of the board, consider printing it as a poster or hardcover book.