How often should you string your racket?
As a rule of thumb, you should restring your racket the number of times you play in a week per year. So if you play 3 times a week you should restring your racket 3 times per year. That includes if you use your racket or not. As the string bed will lose its tension over time.
That's why all professional tennis players have their rackets freshly strung before each match.