Cashback credit is valid for 60 days, counted from the date you place your order, not from the date the credit lands in your wallet.
When the clock starts
Worth noting: since the credit is added after delivery, the delivery days count toward that window. So if your order takes 4 days to arrive, you have 56 days left to use the credit.
How to check your expiry date
Check the expiry date on your wallet page in your account, and use your balance before the window closes.
Your balance renews with every order
And because the credit renews with every order, shopping regularly keeps your wallet stocked with valid balance.