Constant PENDING_SURB_TTL 

const PENDING_SURB_TTL: Duration;

How long a minted SURB stays eligible to be observed coming back.

A SURB the replier never uses would otherwise sit in the pending map forever, so the expectation it raised is retired after this long. Generous relative to a round-trip, because expiring a SURB that is merely slow would count a success as a loss.