LogFAQs > #963556272

I would consider "one sitting" to be one play session during the day, essentially from the first time you turn on the game until you go to bed that night. If someone says they bought a new game on release day, played it all day, and beat it in one sitting, I'm not gonna say "well actually you stopped for an hour to eat dinner so that's 2 sittings".

Although really I can't think of any cases where the technicalities of the phrase are going to matter even the slightest bit. In any common conversation, someone will be able to clarify what their "sitting" was. Even in a speedrun sense, games will have defined rules about how the timer is handled, regardless of many sittings they allow.