In our opinion you should do a backup at the beginning of the month, one at the mid month mark and one at the end of the month.
The reason that we believe that you should backup your data three times per month is to limit the amount of data that can be lost. This also limits the amount of data that you need to rebuild and compile in order to keep your business, and for that matter you life, flowing in the right direction.
Lastly, when you do that backup do not store it on the same media, hard drive, flash drive, or server where you have other data stored. If possible, store it on a system that is not connected to the internet. Believe it or not, that is still possible in this day and age.