It really depends on what kind of strings you're planning on getting and also how often you play.
I play (or at least used to) quite frerquently and I changed them at least once a year.
If you're playing seriously as a soloist, this is my recommendation.
If you're playing at, like, orchestra or something as a side activity, you could probably get away with two or three years since your old strings wouldn't stick out as much.
Also, are you changing them yourself, or having someone else do it?