For profane foods (bread or wet vegtables), a single hand-washing in the morning is sufficient for the entire day, assuming a basic level of awareness.
If a person was mindful of their hands, no further washing is required unless a specific impurity is known to have occurred.*Babylonian Talmud Chulin 106b
Greater mindfulness is required for Terumah and sacrificial meat, due to their sanctity.
A priest pure for Terumah or sacrificial meat, who diverts their attention (Hessech HaDaath) from maintaining purity, without actively guarding against impurity, is assumed to have become impure.*Mishna Taharoth 7:8
They must immerse their entire body in a Mikveh to restore purity, without requiring sunset, as the impurity is rabbinic.
In contrast, if someone had washed or immersed their hands specifically to handle Terumah or sacrificial meat, and then diverts their attention, they must purify their hands again, even if they claim, “I know my hands did not become impure,” because the Sages do not rely on this assertion, as hands frequently engage in activities and may touch something impure unknowingly.
However, if the person has no idea whether their hands remained pure after such distraction, they must immerse their entire body in a Mikveh, as with full-body impurity.*Mishna Taharoth 7:8
If a person did not guard against possible exposure to other impurities, they must follow the appropriate purification procedures, which includes immersion, before they can handle Terumah or sacrificial meat.*Babylonian Talmud Zevachim 99a All immersions related to Hessech HaDaath are rabbinic, reflecting the Sages’ stringent measures to protect sacred food.