Off the top of my head, you have about 16min flight time and can climb about 12 meters/sec so that would be 60*16*12 or 11,520 meters or around 38,000 feet. So let's cut that in half to be reasonable and you have 19,000 ft. Of course, that's using all you power to go up. Coming back down will be _much faster_.
If you want a controlled ascent/descent then I'm going to guess about 3,000 ft. That's just a guess though. The experts here will be able to tell you exactly.
I bet a few of them have even flown that high themselves.