This is a bit of a loaded question. There are two important life expectancies to consider:
1. The number of charge/discharge cycles a Power Bank can reliably perform in its lifetime.
2. How long a Power Bank can retain its charge when not in use.
The answer to point one can differ between models of Power Bank, their internal components and the quality of their manufacturing. We try not to stock Power Banks which have fewer than 500 charge cycles in them. This would allow you to charge a device from the Power Bank every day for a 1.5 years before it started to lose its ability to hold charge long-term. Better and more expensive Power Banks can last longer, while smaller and cheaper units may fall short depending on their treatment. Power Banks are generally not used daily, so they often last much longer than 18 months in real-world usage patterns.
Point two depends on the quality of the controller circuitry and battery cells. A good Power Bank can hold charge for 3 to 6 months with minimal loss. Lower quality Power Banks may struggle to retain a useful charge more than 4 to 6 weeks. In this regard, you get what you pay for, and if you need a long-term emergency power supply consider increasing your budget to ensure you're not going to be caught short. Most Power Banks will slowly lose charge over time, to a degree influenced by the environment and their treatment. For example, leaving a Power Bank in the car where the temperature can fluctuate greatly over time can shorten its lifespan.