How soon will I be refunded if my order fails?

That your order failed can have several causes. How quickly the money is in your account depends on the reason for your failed order and your payment method:

technical failure

iDEAL or afterwards: within 3 working days
Credit card: within 5 working days

transaction expired

with iDEAL payment: topped up within 1 hour. Tip: check the value in your account