Brakes are changes when the wear indicators are shown to be at their minimums. (typically pins flush with the housing with the brakes being set.) There is no time/cycle limit that I am aware of. The caveat is if the a/c is at a maintenance base going out on a long-haul trip or during a higher level check, the brake may be changed before it hits mins to save the hassle later.
High weights, short runways and crew usage will affect the life. No simple answer to this question.