FINDING · DETECTION

A blocked Tor bridge becomes reachable again after approximately 12 hours if Chinese scanners are unable to reach it continuously. In the authors' experiment, one bridge (port 23941) whitelisted to their Chinese VPS via iptables was unblocked within 12 hours despite remaining actively used, while an unrestricted bridge (port 27418) stayed blocked indefinitely.

From 2012-winter-great — How the Great Firewall of China is Blocking Tor · §4.2 · 2012 · Free and Open Communications on the Internet

Implications

A bridge can implement a firewall rule to drop connections from known scanner ASes (AS4134, AS4837, AS17622) or require a pre-auth secret, causing the block to expire after ~12 hours and restoring access to legitimate users.
Design bridge distribution so that a bridge can temporarily go dark to scanner traffic (rate-limit or reject unauthenticated TLS from China-origin IPs) without disrupting users who already hold credentials.