Changing strings once or twice a year... That's amazing.
After three weeks, when cleaning the strings just with a dry cotton cloth, the cloth gets black lines (I'm white and wash my hands everyday and more than once. so... easy jokes are not allowed
). So, strings get dirty.
(I use to play every day for half an hour as a minimum).
I don't know exactly how frequently I change the strings, and more these days that I'm testing different brands and gauge.
I used to apply oil when changing the strings, but that's too much compared to JB comment.
I use Dr. Duck's. It's small, but I bought it more than 5 years ago.