Really, once a week is sufficient, nightly is better with the hardware being super cheap there is no way you can't opt for a $100 USB drive to backup your system. If you don't backup and lose all of your data look forward to getting drilled by the recovery guys, IF they can recover any data.
Best Buy, Frys they all have small drives that you plug-in at night and setup the software to run incremental backups. This means the software will only backup the items that change.
You will sleep better if you even pay attention to the value of your email and stored data to $$$$ and time. If you don't worry and are oblivious of the time lost so be it.
Here is one of the lowest cost and portable drives: Western Digital My Passport Essential 320 GB USB 2.0 Portable Hard Drive WDME3200TN (Midnight Black)
Of for $20 I would recommend upgraded to 1 Terabyte Western Digital My Book Essential Edition 1 TB USB 2.0 External Hard Drive WDH1U10000N
Also if you have a home network plugging in a networked drive large enough will allow all of the computers in the house to backup over CAT-5 or your wireless connection. Check out the Iomega Home Media 1 TB Network Attached Storage 34337
For questions, email me.