I got a measured burn time on low (4,000 BTU/hr nominal) of five
hours on 16.7 oz of propane.

I just did a half-hour burn at high setting (9,000 BTU/hr nominal)
and it works out to about three hours 28 minutes per
cylinder.  Worst-case max (based on scale precision of 0.1 oz) would
be 3:55 and min 3:18.

In Canada 16.5 oz of propane has 22,300 BTU (minus about 200 BTU heat
of vaporization), so that works out to 4400 BTU/hr on low and 6300 on
high (min 5700, max 6700).

If it really delivered 9000 BTU/hr the cylinder would last only 2:28.

This is for my particular sample -- they may not be uniform.

Yours,
David