Function renewal_due 

pub fn renewal_due(
    setting: KeepWarm,
    cache: CacheUse,
    lead_ms: u64,
) -> Option<u64>

The instant the next renewal request is sent, or None when none is.

lead_ms is how long a request to the caller’s endpoint takes from leaving the city to the provider starting it, as the caller measured it. The renewal is due that long, plus PROMPT_CACHE_RENEWAL_MARGIN_SECS, before the lifetime the real use started ends, so a renewal no slower than the lead starts while the entry is still alive. A use whose renewal was sent plans none.