I always change my banjo strings every couple months at least, because when they get too much rust on them playing up the neck becomes nearly impossible I just don’t play guitar enough to justify changing them super often. I have been playing more lately so I might start changing them more often. I’m pretty sure I last changed them in January, so I’m long overdue haha. And mandolin strings sound good for me for a long time, I honestly like the tone when they lose their initial brightness. I change them after they get uncomfortable to play, which is usually every couple months.