There are two categories of backup to concern yourself with–User Data, and Whole Disk. User data is the more critical of the two. These are the files you have created. Your time in creating them and what they contain have value. The value you place on them determines the type of backup you implement. Whole disk backups concern user data plus the operating system of the computer and all the programs you have installed and the custom settings you have painstakingly made to them.
We will deal with each in turn and focus primarily on minimalistic, simplistic backup schemes.
If your data, and the time needed to create it has no value, there is little need to backup. The thoroughness and complexity of the backup are generally directly related to the value of the data backed up.
Data you create should be backed up frequently, possibly as often as you use the computer. A method that simply mirrors your data is simplest and can be very fast. Programs are available that make a copy of everything in your user folders to another location. They only copy files that have changed, so after the first backup, they are quite fast. The files on this backup are generally just copies of the original files and are not compressed or otherwise different from the originals, so they are easy to retrieve if the original is lost or destroyed.
Different software is used to make this type of backup, which usually consists of one large file made up of everything on the hard drive. The purpose of the backup is to safeguard against hard drive failure, or other large scale problem, such as a malware attack on the computer. The backup is usually compressed to save space and the program used to make the backup is usually required to read anything on the backup, making it more complicated to use for a restore. Think of it for restoring after a catastrophe.
At minimum, it’s best to have at least two full backups of both User Data and the Whole Disk. If one of each of these can be kept offsite you have the additional protection of loss through fire and similar. One reason to have two backups is that if a major problem occurs during the backup, destroying the content of both, there is still an additional copy of the files available. See above comments on data value.
Only you know how valuable your data is and so only you can know the frequency, number, and location of your backups. Here is a suggestion for the average, light user.
Whole disk backup. If you update or add programs regularly, a whole disk backup should be made monthly. If changes to the system and programs only happens occasionally, then quarterly backups could be sufficient.
User Data. As already stated, this should be backed up as much as daily, or at whenever you do significant work on the computer. By significant work we mean copying over pictures from your camera, adding an hour’s worth of data to your financial program, etc. If daily is too much to handle, then at least weekly.