In regards to kosher supervision, what is the largest single gap that a mashgiach can leave the premise and still be considered practicing יוצא ונכנס?
From the Kashrus Organizations, it sounds like it depends on what product the Mashgiach is being יוצא ונכנס for. At least for the milking process, the time frame for the Mashgiach's entering/leaving given by the OU is
"In general, Poskim have ruled that yotzei v’nichnas supervision requires the mashgiach be present at the beginning of the milking, come in unannounced at least once in the middle, and come back at the conclusion of the milking. This applies to each milking session, typically lasting eight hours or so."
If you divide the 8 hour duration into beginning, middle, and end, then it's 4 hour intervals.