eh by durable i mean that the strings don't rust that easily?
there's a difference between 'rust' and 'dark spots'
also depending on how you maintain ur guitar
if you wipe it down everytime after you play, and wash your hands before you play, you may find that strings last alot of longer
i have sweaty palms, so usually after 20-30 min of playing, ill have to do a quick wipe