[Stormcrow]

It depends a lot on the instrument and what you're playing. I play the horn, and I can last a lot longer playing quiet classical pieces than I can bombastic John Williams arrangements. With the latter, the lips become too fatigued to keep up the high notes. I find oompahs in marches to be fatiguing, because all those short notes don't let me exhale much, so I'm essentially holding my breath through each march.

Years ago, when I played the flute, the problem was not fatigued lips, but lightheadedness from too much exhaling, and losing circulation in my fingers from holding my hands up for too long. But the type of music didn't have as much impact.

But in general, several hours at least. A professional could no doubt go much longer. If it's particularly fatiguing music, though, the time is cut down a lot.