I doubt you will find a standard answer for this question as most every company has a philosophy on starting/stopping antifoam injection based on their specific unit and coke production. These parameters change over time as crude slate changes occur within your refinery. In experienced units/operating crews they will adjust injection to save on chemical costs as well. At Chevron we trend every drum to watch various parameters to ensure we are not entraining particles and carrying over to fractionator.
If you don’t care about chemical costs and you want to be conservative then start the injection soon as you see the first indicators of antifoam being needed and just leave it on until drum is at the level you desire.