Answer
The answer is: 5 years. Şahin returned to Borussia Dortmund after his loan spell at Liverpool in the 2013-2014 season and stayed there for five seasons until the 2017-2018 season. During his second stint at Dortmund, Şahin made a total of 131 appearances for the club, scoring 11 goals and providing 20 assists. He was an integral part of the team that won the DFB-Pokal in 2017.