Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Severe breathing difficulty, blue or gray discoloration, confusion, fainting, or severe chest pain requires emergency care rather than waiting for a scheduled appointment.
Follow-up depends on the cause and clinical setting.
After surgery, the care team may monitor oxygen levels, breathing, mobility, cough strength, and pain control.
Improvement may be judged by symptoms and examination rather than repeated imaging in every case.
A follow-up chest X-ray or CT scan may be recommended when atelectasis was extensive, does not improve as expected, keeps returning in the same area, or has no clear cause.
Further evaluation may be particularly important when an airway obstruction is possible.
Routine follow-up is appropriate for reviewing recovery, test results, smoking cessation, or management of an underlying lung condition.
The article also says
At home, follow the activity, breathing-exercise, wound-care, and equipment instructions provided by the care team.
Contact a clinician if breathlessness is not improving, exercise tolerance declines, fever develops, mucus increases, or symptoms return after initially getting better.
Every sign above is quoted from the sofidex article on Atelectasis. If you are not sure, err upward — it is always reasonable to be checked.