Six weeks is enough to get anyone frustrated but the problem is that the drain has produced a hole in the lining outside the lung called the pleura. The effusion, or fluid, lies between pleura and lung. Because this space is normally at a negative pressure there is a risk of air being sucked in which results in the lung collapsing.
We would not allow a patient to fly for several weeks after a drain unless accompanied by a doctor equipped to reinsert the drain in flight. If you are crew, a longer period is necessary, and although there is no published data on what time period is safe, six weeks is hard to argue about.
The cause of the effusion also needs to be taken into account, depending on what the cause is.....
Hope this helps