After how many updates should a vacuum be performed?

We have a database that periodically we perform large updates, around
a million records, after that the vacuum will run for 12 hours without
completing.  After that, I typically remove the 2 indexes and 1
constraint on the largest table, 7 million records, and the vacuum will
complete in a couple of hours and the indexes can be recreated in a half
hour.

After how many updates should a vacuum be performed?  Do indexes need to
be recreated periodically?  Would auto vacuuming help in this case?  Any
suggestions on tuning?

The database is in PostgreSQL 8.0.7.

Thanks.

Ellen