Six weeks perhaps seems quite long - It sounds like the pleural effusion was reactionary to pathology around the liver post-op (not uncommon but requiring a drain is slightly unusual). I also presume that the drain was a small pig-tail type (diameter not more than a few millimeters) rather than the traditional chest drains (more like a 1cm tube in diameter). So the defect in the pleural would be small. I also presume that they have plans to rpt the chest film to show that there has been no re-accumulation of the effusion.
CASA (regulatory authority in Australia) give no guidance on time off after a pneumothorax but state that it doesn’t preclude recertification. When we put the large tubes in for trauma/cardiac surgery we advise no flight in pressurized cabin for at least 10 days only – which is in line with Qantas’ fitness to fly policies – but that is as a passenger.
Recon you should speak to the CAA wrt getting an accurate time off flight duties – It may all be arbitrary anyway if you are flying at low level in a single without pressurization (below 8,000).