Well, that all depends on you. It’s usually related to how often you play. If you’re playing year rounds we feel the minimum is about 4 times a year. Any less than that and you’ll miss out on some feel and power you might normally have with a taunt racquet.
It might seem obvious, if you play a lot you have to restring your racquet more often. If you’re playing 5 times a week you might have to restring 5 times a year. And if you’re playing 7 times a year that number might be as high as 7.
If you only play during the summer (or winter months depending on where you are) you should restring your racquet before your first match and every three months after that.
The cost to restring your racquet really depends on your local area. It might cost more if you’re in a large city like London or New York than a smaller town. A quick Google search should yield some results in your area, also don’t be afraid to ask your Tennis partners or the Tennis pro at your local club. Keeping your racquet properly strung is an important part of your game, think of it like training.