how high can you fly in an unpressurised airplane before you pass out?

Answer 1

Just above 10,000 feet.

Answer 2

It depends on a lot of things. One being your age, your health, time of day, density and temperature of the ambient air.
There is a chart called time of useful consciousness.
It basically will tell you, and average adult, how much time you have at a specific altitude before you pass out. Keep in mind that this altitude is based on the standard international atmosphere.
It varies. At 20,000' you may have up to three minutes.
At 36,000' you may only have seconds.