The 2 capacitive buttons next to the Home button on the Galaxy S6 are called touch keys and function as the Recent App key and Back key. To change the touch key light duration on Galaxy S6 do as follows:
1. Open Settings and go to the Device tab
2. Touch the Display option and tap to change the Touch key light duration option
3. You have 4 options to choose from: 1.5 seconds, 6 seconds, Always Off, or Always On
** Note that, if your carrier for any reason has disabled this option for you, you can manage the time duration for keeping the touch keys light on by installing Galaxy Button Lights form the Google Play Store.