Does anyone know the particulars on how expiration works.

Normally we are run expiration with a time limit or 6 hours.  When we ran it
without the time limit it ran for 15 hours and never finished.
Does expiration start reviewing the db from the beginning or is there some sort
of flag set on where to start.
I'm trying to figure out if we never let it run through completely will I ever
get back all the data that has expired or not.

Thanks for the help