My general rule is - if I didn't use it for a year, I don't need it. There are obviously some exceptions like a fire extinguisher (which I hope to never use) and digitized photos, which only go through a careful selection.
I think the thing I kept the longest was a Libranet Linux 3.0 CD set because I worked for Libra Computer Systems for a while and this was the release that I helped building. A few years ago I threw it away, I think after I saw someone uploaded it to archive.org. When I'm 60 and want to install it again for good old time's sake I can.
tl;de: if you don't use something for a year, you probably don't need it.