Looks like the Hammer Nutrition literature has some good things to say about protein consumption for rides over 2 hours but not much for under it.
All athletes that are involved in ultra-intense, ultra-long exercise sessions, have a "protein horizon"
where the metabolic processes requiring dietary protein intake become as important as ingesting carbohydrate for energy production.
This
"protein-intake-horizon" is dependent upon the athlete's personal level of training, the intensity of his exercise, and the length of time spent at the particular exercise session.
Since every situation is different, there is no good way to express a "golden rule" regarding protein benefits during exercise. But, in any case, at some time, whether it is 2, 6 or 8 hours into an event, all athletes will begin to receive more physiological benefit from a balanced diet than from strict carbohydrate ingestion. My own experience suggests about six hours.