Switch from sleep to hibernate after set time
Retina MacBook Pros and newer MacBook Airs actually do that by default under certain conditions. If Power Nap is disabled, the computer is on battery power, not connected to USB devices and not paired with Bluetooth devices, it enters standby mode (hibernation) after about 70 minutes of normal sleep.

If pmset -g | grep standby looks like this, your computer supports standby mode:

 standby              1
 standbydelay         4200


You can change the delay to 15 minutes with sudo pmset -b standbydelay 900.