When do I Vacuum ?

hi,

I've got a DB driven web application which currently has approx 298 inserts
per day and 540 edits/deletes.

This figure will change daily and currently i have a cron job set up to
vacuum the database every hour during 'work time':

0 7-19/1,23 * * *       /usr/local/pgsql/bin/vacuumdb -z -d edb
30 7-19/4,23 * * *      /backup/dumpit

I've also started reading Bruce's book (and what a fine book it is ... :-)
and noticed that he suggests vacuuming once a day when the db is quiet.

Would this be sufficient ? The database drives a web based
calendar/appointment booking/reporting system i've written in php and is
used extensively by 3 departments and has approx 200 people log in each day.

Needless to say speed it paramount but I've got a sneaky feeling I might be
vacuuming too often ....

Many thanks for you help,

Steve