You an use current or potential as the standard reference. set a value X, while I or V > X, it is the start point, and while I or V < X, it reaches the end. It is good to have the rule for the end point, and you can take the start point as where you inoculate your reactors.
During batch cycle, when you feed the reactor with influent is the begin of cycle. At the end, when u removed effluent is the end of cycle. Its depends upon how fast microbes utilizing that feed.