Beste antwoord door Robert B
It (NS Flex Dal Voordeel) will show up in your Mijn NS account within an hour or two.
Don't forget to load it (Reizen op Rekening) on your card, see here for where you can do that (choose pick up order/bestelling ophalen).
If you ordered NS Flex on a new card, the starting date is always 7 days after ordering. Your 'Mijn NS' account will show it by then.