Some notes on my reasoning:

I was originally going to guess an answer of 10000 feet on the basis that I vaguely remembered a Nickleback song indicating that commercial airplanes go up even higher, but thought without looking that the lyrics might have been "join the mile high club at 20000 feet" and so disregarded this answer as biased (but apparently the lyrics to "Rock Star" are "37000 feet" instead, so this was a mistake). I then figured that 6x the height of the Sears Tower would be a somewhat reasonable height for a mountain, and thought that the Tower was either 300ft or 600ft tall, eventually deciding on 600ft (actual height: 1451ft). 6(600) = 3600.