Can you tell me how long to recover after a subacromial decompression on my shoulder?

Really depends: Many different procedures can be done to the shoulder during an arthroscopic surgery. If bone spurs removed, shoulder debrided, then pain may be minimal and pass within a few days. If cuff repaired or labrum repaired, pain can persist for a couple of months. Make sure you understand exactly what is being done during your scope. Don't compare your results with someone else's.