Does the calculation of the 'iddah depend on the bleeding lasting for a day or two, or must the menstrual period continue for its usual number of days?

The blood after postpartum bleeding (nifas (definition)) could potentially be either nifas blood or menstrual blood (hayd). If it is nifas blood, it does not count towards the waiting period (iddah (definition)), and the iddah begins after purification from it. However, if it is menstrual blood (after purification from nifas or after forty days), and if the period between the two bleeding episodes is seven days—as you mentioned—then this returning blood is considered a continuation of the previous menstrual period and is not considered a separate menstrual period. Thus, it remains a single menstrual period to complete the iddah.