Guitar strings don't age well. After a few weeks they start to go a bit dull and dead sounding, no matter how much you try to clean them. That is because all the sweat and dead skin from your fingers (not to mention the grease from the bowl of cheesy chips you had between the sound check and the gig) all gets lodged in the grooves and coils.
I tend to change my guitar/gazzouki strings on the instruments I gig with, every 8 weeks or so and clean them every time I play them.
However most bass players aren't that picky, most tend to change them every ten years whether they need it of not. They usually claim that they prefer them more dull and mellow. I change my bass strings about once a year which gets a bit expensive with 7 basses, as good bass strings tend to cost about £30 a set.
