I know that strings eventually deteriorate with time. It looks like my emergency spare set of strings (nylon) is 5 years old. How old is too old? I change the strings on my harp every year or two but my spare set has been there for a while. 🙂
Nylon strings last longer (assuming they’re kept in a comfortable environment, not exposed to extreme temperatures) than gut strings. They should still be fine. If your harp is normally strung with nylons, use them next time you change all the strings and order a new set for spare. Then you can rotate each time you change them all.
I always use from the spares and then replace spares with new strings, as Catherine has said above.
You didn’t mention how you keep your strings.
I keep each octave in its own plastic ziploc, all the nylons together in their ziploc, the guts in their ziploc, and the wires in their ziploc. So they are all double-bagged as well as being in their original little ziploc. (triple-bagged, really) Spares at home are in a protective canvas bag. Traveling spares are in a nylon bag.
I have two harps, so I have a double spares supply, but I use out of the traveling spares only….and then replace those with strings from the bag of home spares. I write the date that I got the string on masking tape and tape it to the ziploc. Some of mine (5th As and Bs) got to be 20 years old before they were used, and they were fine.
I never replace all strings at once, only what’s dreadful. I don’t have the time or money to do that.
If you’re getting rid of 5-yr-old strings, send them to me.
Here is my website to listen to my old strings. http://www.WeddingHarpMusic.Yolasite.com