I ran an exit node for a while, and found myself auto-banned from so many services that I stopped running the node and threw away my IP range (which now would be worth $$$ - oh well!)
This also means the expiry time is usually tied to however long a Tor exit node stays on the DNSBL + 3 or so days (depends on how long the software is configured, but 3 days is typically the assumed default for IPs that tend to get mixed up with automated spam, of which Tor is also a massive purveyor.)