"When should I change my strings?" That's a question we often hear. New strings usually sound better, but there are as many answers to this question as there are musicians. Some things that you can consider are:
1. There's no 'official' time to change strings. I used to change the strings on two guitars and two mandolins every day when we played bluegrass festivals, but Bill Monroe changed his strings once a year-- at New Year's-- and from then on, he just changed them when they broke (which was pretty often, by summertime).
2. Some people like the sound of old strings. Our Cousin David loves the sound (or lack of it) that old strings have, and would probably prefer never to play on new-sounding strings. I think that brand-new strings can sound a bit tinny, myself, but sometimes-- such as when I have a big stage show to play, or a noisy party gig or bar gig where there's going to be plenty of musical stress and challenge-- I'll make sure at least that my strings aren't too old.
3. Generally speaking, newer strings make your instrument get in tune (and stay in tune) better. This is because (a) a new string isn't worn from playing and is still about the same diameter from one end to the other, so it "frets" more in tune; (b) the string is not very corroded yet, so it slides through the nut-slots and bridge-slots more smoothly as you twist the tuners; and (c) the lack of rougher, corroded surfaces on the string make its vibrations more coherent so you (or your electronic tuner) can hear the string's note better. Also, new strings (or preferably a day or two old. so they're "stretched" and stable) are usually better for recording, because getting exact tuning, and having the strings stay there, is really critical if you're in a recording session.
. . . . .
So those are some things you can think about.
Editor's Note: For even more detailed info on this topic, you can see Red's previous post on this same topic.