The useful lifespan of solar is roughly 25-30 years. In a 2012 study by the National Renewable Energy Laboratory (NREL) found, on average, output of your solar panel falls around .8% each year. This decline in output is called solar panel degradation rate. The rate of degradation varies slightly by brand, and premium panels can be as low as .5%.
 
This means, that if your panel degradation rate is .8%, after two years, your panels would operate with a solar output of 99.2% of their original output. By that calculation, even after the "useful lifespan" would still be operating at 82.5%.
 
Solar panels offer different warranties to cover both equipment and performance warranties. The performance warranty, typically around 25 years, will give you sense of the electricity output expectations of your system over time.