Well it depends on 2 factors really:
1) What type of string you use (and)
2) What tension you use them at.
1) If you use real gut strings, thin type, they'll be likely to snap sooner, than the other extreme, which are extremely thick strings (everlasting
2) If you play with a heavy racket tension (control players usually do, as opposed to power players who want as much power from the tension as possible) they'll also be more likely to snap sooner; cuz they are tightly strung.
Basically a Factor 3) as mentioned already in this thread, is the amount of time you play of course. Once a year for regular tennis play is average I was told.