I'm pretty sure the amount of cyanide varies widely from one airplane
fire to another, but there is no time to measure it.

as I understand it the HCN is produced when plastics containing
nitrogen burn in an oxygen poor environment. Stuff like synthetic
rubber upholstery, pulyurethane foam insulation and and melamine
tray-tables

As I understand it, this is akin to the major reason you're supposed
to get out of a computer room if the Halon extinguishers are
triggered. The Halon itself isn't particularly hazardous (at the
concentrations used in these systems), but the combustion byproducts
from burning plastics and etc. are really nasty. The Halon suppresses
some of the flame reactions and stops the fire, but it doesn't get rid
of the poisonous partially-combusted plastics and other decomposed
flammables.