If you are a beginner or play just occasionally, a maximum of one game a week, you could play with the same racket even up to four years if you take good care of your racket and the frame does not break.
If you play on the higher level and you end up playing about three games every week, you may need to replace your racket every six or eight months.
You should not play with a dirty or scraped racket. Or with one that is in bad shape. Sometimes professional players change the racket because it influences their performance, but there might also be some esthetical reasons behind this as well. A new racket is a new racket.