Ammonia Reaction with O2
No.  Because remember, the oxygen is being consumed too.  If we assume it's reacting via that reaction (realistically, you're going to get a complex mix of products) then we start with something like: 30% NH3 56% N2 14% O2 (the mix is 70% air, which is an 80/20 mix of N2 and O2).  For every 4% reduction in Ammonia, you're getting a 3% decrease in oxygen.  So O2/NH3 reactions will be rarer (slower), and slow down heat generation.  Also, the amount of inert gas in the vessel will be increasing, which also need to be heated up, so less of the enthalpy of reaction will go into increasing the temperature.

That said, above the autoignition temperature, it's going to be a pretty fast reaction anyway...  What we can be sure of is it will stop reacting at about 10% NH3 when all the oxygen is gone.