Monitoring Streaming Replication in 9.2

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?