The program, which began in April 2014, first covered iPhone 5 models for two years after the first retail sale of the phone. That has recently been extended to three years, per Apple's support page:
The program covers iPhone 5 models for 3 years after the first retail sale of the unit.
This program allows qualifying iPhone 5 customers with faulty sleep/wake buttons to get them repaired free of charge. Problems with the sleep/wake button have been a common occurrence for many iPhone owners since the launch of the iPhone 4.
Source: Apple
