If you want to check that something is constant for a period of time you can usually do it very simply like this pseudo code - which is the same as the logic proposed in Reply #1. It may seem upside-down-logic because it updates the time when the person is not present.
if (personNotPresent) {
lastTimePersonNotPresent = millis();
}
if (millis() - lastTimePersonNotPresent >= 6000) {
// person has been present continuously for 6 secs
// do something
}
...R