I propose the hydrogen cyanide - fluorine rocket.
It has the fabulous potential for a vacuum Isp of up to 380s, with a propellant mix density of 1023 kg/m^3, handily beating methalox mixes. It is non-cryogenic and burns at over 4000K.
Exhaust is 43.3% hydrogen fluoride.
my guess, with rough guesses via gut math? probably no longer than 5 minutes, with an ablative teflon coating. maybe longer if fired in short bursts to allow for ablated material to cool, but definitely not safe to fire within atmosphere.