There are those players, who don’t play a lot or don’t hit the ball with much topspin or don’t play on HarTru, and their strings last a long time. For those of us who do some or all of those things, their strings will break in a couple of months.
A good rule of thumb for restringing time is you should restring your racquet as many times per year as you play each week. So if you play four or five times a week (with one racquet), you should restring about that many times per year – or, roughly every two to three months.
But if you do not want to be “surprised” on the court during a big match, watch out for “notching” that takes place where your strings rub against each other. Once you see those grooves starting to develop, a break in the strings will not be far behind.
And if you want to minimize that notching – and have your strings last longer – you can go to the extra step of inserting some type of “string savers” as soon as you get your racquet restrung. Several companies make them; but I have been using Gamma String Saver Deluxe for a number of years. It takes about 15-20 minutes to insert them into the string bed; and can be done while watching some tennis on TV.
In addition to helping your strings last longer, the little plastic inserts provide a couple of extra benefits: they help the strings slide back into place (so you are not constantly fiddling with your fingers to do that); and I believe they provide an extra rough surface to impart a little extra spin on the ball.
And even though the picture shows an “illegal” placement of a dampener, the use of “string savers” are specifically ALLOWED under the rules.