How long to keep things is very personal.
I would suggest coming up with a policy for backups.
For example, companies do this, and do something like ...
Make a daily incremental backup at 12:01 am
Weekly on Sunday at 7:01 am make a weekly full backup
and delete daily ones older than 2 months.
Monthly on Sunday at 11:01 pm make a monthly full backup
and delete monthly ones older than 24 months.
Two copies might be stored on drives kept off site in 2 places.
Yearly on Jan 2nd make an annual full backup on permanent media
(e.g. dvd) and keep 2 copies in different places forever.
The older the file the harder to get to and restore.
If a file is real old and kept for a short time is might be lost.
i.e. A file existing for less than a year might be lost after 2 years
and for something that was edited and deleted before the
next monthly backup the final edits might be lost after 2 months.
Obviously a personal policy would not be nearly so complicated but you get the idea.
Maybe -
Every six months make a full backup on dvd and keep forever.
Every 6 months go through the list and keep everything from the last 6 months,
beyond 6 months keep only the 1st ones done each month for the last 2 years,
and delete all older than 2 years (except the 6 month dvds).
It is easy to make a policy.
The real hard part is, like new year's resolutions and diets, sticking with them.
It is easy when every thing is running smoothly to let things lapse.
Then when there is a problem - oops.
