The answer depends on the reason you made a backup in the first place.
Let’s look at some of the reasons, and the implication of each on what’s more formally called “data retention”.
Types of backups
I think about three general classes of backups:
Let’s look at each of these in turn.
Some backups are made simply as a safety net prior to some risky event. In case something goes wrong, you can restore your system to its status before that risky action.
For example, you might back up your registry prior to installing software you don’t completely trust. Another example might be a system-image backup of your entire computer, taken just before upgrading the operating system.
These types of backups are often temporary. Once the risk has passed, and you’re certain you’ll never have to revert to that backup, there’s no need to keep it.
That could be moments, hours, days, or weeks, depending on what it takes to feel confident that you’ll never need to revert. Typically, it’s closer to days than weeks.
I consider regularly scheduled backups to be the most important way to protect yourself from data loss.
My general recommendation is to use backup software to automate a monthly full-image backup of your machine with intervening daily incremental backups. The specifics – monthly and daily – are less important than actually having something that happens automatically, with no need for you to remember and take action.
There’s no set answer as to how long you should keep these, as it really depends on your own configuration, needs, and storage capacity. You might discard backups older than a month, or perhaps a year. You might decide to keep specific snapshots for longer, “just in case”, but discard the majority.
As just one example, here’s my retention schedule for backing up the PC I use as a primary work machine:
- I keep daily incremental backups for a month, until the next full backup occurs.
- I keep monthly full backup images for at least three months.
- I keep the full backup images for each quarter for an additional year.
- I keep the first full backup image of each year pretty much forever.1
As I said, that’s just an example, and my needs might well be considered extraordinary compared to yours. (I use something from my backups perhaps once a year or so. Totally worth it.)
I want to mention one additional type of backup that many might not consider to be a backup at all: what I call an archive. An archive, to me, is a collection of data is intended to be kept forever, even though it’s not necessarily needed now, or needed daily. For example, those backups that I keep “pretty much forever”, above, might be considered archive copies of the long-defunct machines they represent. Similarly, the fact that I copy my photographs to cloud storage, in addition to backing them up locally, might also be considered archival.
The concept of archiving is truly data-dependent. There’s no need to archive your operating system updates for posterity, but your correspondence, photographs, and other more personal items might be appropriate for archival.
A rule of thumb
A good rule of thumb is to simply think long and hard, “Would I – or anyone – ever actually need anything from this backup ever again? And for how long might that need exist?”
Then keep it a while longer.
Before answering that, we also need to look at what’s been backed up, and what the implications of “needing it again” might be.
Needing a backup
A complete system restore to a backup image resets everything on that machine to the condition it was in on the day the backup was taken.
Everything since the time that backup was made is lost.
For example, perhaps on September 1, I restore my computer to a full image backup that was taken on June 1. All changes between June 1 and September 1 (that haven’t been saved elsewhere) are lost.
As you can imagine, then, while I might very well restore to an image of a few days ago because of a system failure or other catastrophic event, I certainly won’t be restoring my system to the image taken on January 1 two years ago.
Those images are valuable because of the files they contain. While I might never completely restore my entire machine to their contents, I can still use my back-up software to explore and restore specific files from the backups taken on those earlier dates. And because they’re image backups – backups of absolutely everything – I know that anything on the machine at that time can be recovered.