The lifespan is the time since cumulative devices sold reached the current total retired devices. The cumulative retired devices can be calculated as 2.05 billion cumulative sold minus 1.3 billion active or 750 million. The time when cumulative devices sold reached 750 million was the third quarter 2013. The lifespan is thus estimated at the time between now and Q3 2013 or 17 quarters or about 4 years and three months.
Dediu notes that lifespan is proxy for user satisfaction and loyalty and speaks to the future rather than the past. Take a look at the chart below for more details or hit the link for the full explanation on how lifespan is calculated.