Do you usually perform a daily backup of websites of small local business ? Isn't a monthly backup more appropriate ?
We do daily backups, just because there's not really a big difference between daily or weekly or monthly except a little change in the crontab and the amount of data that's lost in the case of a problem. :-)
Database dump, and then the whole directory where the site + dump resides gets rsnapshot'ed to another server. Takes a few seconds each night, doesn't cost me a dime, but I can sleep at night.
A backup that requires any manual action should be reconsidered. Automate everything, and use the available tools (rsync, etc.) and you'll be able to do daily backups without any sweat.
Your customers will like it.
The rhythm of backups depends of the rhythm of the website update : new content, updated content, scripts updates... The less resources consuming is to apply different delay to your website parts.
For example a page with a photo is added 2 times a week you can do a database, and images folder backup every 3 days. You update script plugins every month, you do a backup of your plugins directory every month.
There are tools like Website 2 Backup to do automated, encrypted and selected website files and databases backups stored on email, ftp, or on web hosting.