How much context should an NLP system actually keep?

I’ve been thinking about how much conversation history an NLP system really needs before extra context starts becoming more confusing than helpful. Keeping everything seems useful at first, especially when a user refers back to something mentioned much earlier. But older messages can also contain outdated information, changing preferences, or details that are no longer relevant. I’m curious how people working with NLP systems handle this. Do you prefer keeping a long conversation history, summarizing older context, or selecting only the parts that are relevant to the current input? I’m especia