This depends on two factors. First is the quality of the material. When looking for a bag pay close attention to the small details of the make of the pack. Stitch quality, the thickness of the fabric, and the overall sturdiness of the item are important indicators as to how well the bag is going to stand up to a beating. Also, pay attention to how water resistant the bag you want is. Most high-quality modern athletic bicycle backpacks now range from being water-resistant to water repellant. Having this feature is important if you ever get caught in a rainstorm.
The other factor that determines how often you will have to replace your backpack is how you use it. If you barely take your back out of the closet, it may be very low quality but will last years. Conversely, a high-quality well-loved bag will eventually fall apart, although with good gear this should still take a few years to happen.
The best way to really see how quality the backpack you want to buy is to see it for yourself. If possible go to a local outdoor retailer and see what the bags they have really looked like. Holding a few different bags and trying them on is a useful exercise even if you are still going to buy somewhere else. If you don’t have an outdoor retailer nearby or are disappointed in the selection you found, looking up reviews online can be very useful. Both professional and buyer reviews offer insight into the quality of the product and give ideas on new places to look.