No, you need to create an index that matches your query terms. See: http://www.postgresql.org/docs/9.1/static/indexes.html

You can try this :

delete from agent_log t1 where WHERE timestamp < now() - interval '7 days') and timespan < (select max(timestamp) from agent_log t2 where t2.class=t1.class and t2.hostname=t1.hostname and t2.id_address=t1.ip_address)

I suppose you have an index on timestamp and the other fields? A surrogate key might help, or even using timespan as primary key, if possible.

If you can create a history table, then you could:

SELECT
    class,
     max(timestamp) as timestamp,
     hostname,
     ip_address,
     promise_handle,
     promiser,
     promisee,
     policy_server,
     promise_outcome
FROM agent_log
WHERE timestamp < now() - interval '7 days'
into history_table ; 

delete from agent_log
WHERE timestamp < now() - interval '7 days'
[download]

That would be a good start towards partitioning your data, which you might need to consider.