You should choose your string and tension with at least as much care as you choose your racquet. The “rule of thumb” is that you should re-string your racquet(s) as many times per year as you play per week (unless your strings break, of course). As your level advances, you may find advantages to stringing more often so your strings are “fresh”.
When strings go “dead,” they don’t lose power, they lose tension. This means that you have looser strings, so the ball goes farther for the same effort, with less control. You also lose control because the strings feel differently than they do when they’re fresh, so you either have to adjust your game, or content yourself with not playing your best. String facts