Back ups with High Availability Web Server

Here's a question I have a customer interested in.

They have a web application that they make available to people during
normal business hours (gov't entity). It's basically looking up old land
records, court cases, etc. Scanned and redacted documents.

They now what to make this available to anyone. You would pay for 24 hour
access (w/ CC), then so much per page of each document you want to save to
PDF.

The issue they are having is because the number of documents is so large,
the weekly system save they do takes about 18 hours. They do daily backups
that saves only things that have changed, and takes about 2 hours.

They are trying to figure out the best way to do these weekly system saves
and still give the customer a good experience. For example, if they sign
up at saturday at 1am for 24 hours and then backups start for 18 hours,
there's trouble. So they're trying to find the best balance between
availability with the safety of the full system backup.

I said since they are doing the saves every night, maybe they could push
their system save to once a month. Then set up a schedule so that if a
user purchased 24 hours close to a backup time, they are warned and their
time extends past the backup.

Any ideas? I'm sure others have dealt with this before. Thanks!

Brad
www.bvstools.com