263Although there are no rules as to the promptness with which a creditor must act in order to enforce its claims, hypothetical private creditors cannot be expected to demand that the undertaking be declared insolvent as soon as it fails and not to take any account of its longer-term potential, even if it is not, however, acceptable for the public authorities passively to allow debts to be run up over long periods without the slightest prospect of improvement (see, to that effect, Opinion of Advocate General Mischo in Spain v Commission, C‑480/98, EU:C:2000:305, points 36 and 37).