Constant DEFAULT_ABSENT_TTL 

pub const DEFAULT_ABSENT_TTL: Duration;

How long “that repository holds nothing at that key” is reused, absent configuration.

Reasonable range: 30 to 900 seconds. Shorter than DEFAULT_PRESENT_TTL on purpose: absence is the common answer — almost nobody has written an AI preference, and most posts carry no reply gate — and it is also the answer most likely to stop being true, because writing the record is exactly what a person does when they decide to be left alone. Holding an absence for as long as a value would make the moment somebody asks to be left alone the slowest thing this reader notices.