When did the Roman Army decline?

[Flavivs Aetivs]

I reccomend you acquire a copy of Pat Southern and Karen Dixon The Later Roman Army. Until then I will answer as many questions as I can.

There was very little to no decline in the quality of the Army, contrary to popular belief, until after about 406 AD. That's when you begin seeing the first stages of decline.

The Limitanei were not Militia units. They had a tendency to settle where they were stationed, but they were effective military professional soldiers.

Numbers in the late era are exaggerated - at the end of the 3rd century you see an increase to approximately 420,000 soldiers across the entire Roman empire. By the late 4th century with the necessary changes having been made to make a more effective and more suited army to the time, there were about 210,000 soldiers: 90K in the West and 120K in the East (I cite AHM Jones and Peter Heather on this one).

50,000 men at a single battle is highly unlikely - records of emperors like Constantine having over 100,000 men are exaggerating. The largest battles of the era could barely field armies of 50,000 - Aetius needed years of preperation to field his army at Chalons.