Need Help with reindexing

Guys,

I need some help to decide when i should reindex the indexes in the table.
Postgresql 7.4.5 and linux O.S.
Below is the results of vacuum analyze verbose on the table

INFO: vacuuming "public.rpt_metrics"
INFO: index "rpt_metrics_date_network_advertiser" now contains
102960776 row versions in 717963 pages
DETAIL: 198809 index pages have been deleted, 20000 are currently reusable.
CPU 30.95s/3.51u sec elapsed 770.69 sec.
INFO: index "rpt_metrics_date_network" now contains 102960776 row
versions in 517246 pages
DETAIL: 179291 index pages have been deleted, 20000 are currently reusable.
CPU 21.11s/2.69u sec elapsed 917.34 sec.
INFO: index "rpt_metrics_date" now contains 102960776 row versions in
516546 pages
DETAIL: 193753 index pages have been deleted, 20000 are currently reusable.
CPU 20.31s/2.70u sec elapsed 975.39 sec.
INFO: "rpt_metrics": found 0 removable, 102960776 nonremovable row
versions in 2920718 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 56377053 unused item pointers.
0 pages are entirely empty.
CPU 123.67s/17.41u sec elapsed 3132.89 sec.
INFO: analyzing "public.rpt_metrics"
INFO: "rpt_metrics": 2920718 pages, 3000 rows sampled, 176302
estimated total rows
VACUUM

When should i ideally reindex this table ?.
(once in a month, weekly....etc)
--
Best,
Gourish Singbal