Monitoring Streaming Replication in 9.2

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Newb question here. I have streaming replication working with 9.2 and I'm using Bucardo's check_postgres.pl to monitor replication. I see that it runs this query on the slave:

SELECT pg_last_xlog_receive_location() AS receive, pg_last_xlog_replay_location() AS replay

That returns hex, which is then converted to a number in the script.

My question is this: what does that number represent? Is it just the log position? If so, how does the log position translate to queries? Does one log position = one query? (I did say this was a newb question.)

How do I determine a meaningful alert threshold for that value? Is there a reliable way to monitor replication lag in seconds? How do other people handle this?