I don't know anything specific about a collapsed lung, but from my own experiance with other health issues, I'd suggest being very conservative with the timeline for when you start playing again. Your doctor should be able to tell you when you are ready for excersize and other normal exertions, and if it were me, I wouldn't jump into trumpet playing until you're sure you're 100% healed. You don't want to risk hurting yourself, another week or two off the horn won't make much difference in the long run, and restarting your trumpet playing is easier when you're fully healthy. Last year, I think I set myself back for awhile be starting to practice before my whole body was really ready for it. My layoff was MUCH longer, so it may not be an issue for you, but a little caution can be better in the long run.