It's recommended you replace your bra every six to twelve months. However, the life span of a bra depends on how often it's used. One of the best ways to extend the life span of your bras is to have more bras in your rotation. This decreases the amount of times each bra is worn, thereby extending their lifespan.
Some signs it's time to replace your bra:
The underwire has poked through the fabric and is stabbing you
The band has stretched so much that it feels loose on the tightest hook