Less a year than a process. People didn't awakened one day and saying themselves "Well, today the Roman Empire is no more".
If collapse is the failure of roman state to control the territories that composed the empire and/or the migrating peoples in the Romania; where the survival of WRE becomes unavoidable I would say the collapse of Roman Africa.
The collapse of Rhine border, which migrating peoples can cross without being blocked by Romans and settle on their own territories that they rule directly.
But while fiscal ressources are weakened and whole regions escape imperial authority, the situation isn't totally catastrophic, with Flavius controlling the situation whom issues stand periphericals.
The fall of Roman Africa, definitely prevent WRE to survive on its own, as it loose mediterranean domination, fiscality of a preserved region and ravitail or help outside ERE that prooved much interested to meddle as possible in Italy, worsening a situation already unstable, due to higher pressure on Italians.
So, more or less : 429 as the beggining of collapse of the empire in West that would end (symbolically) in 476.