Go to the stronghold security (2nd level). When you enter the room, start the 15 minute countdown. If you leave the room, the countdown will reset. You can move and attack the zombie's and flesh eaters and it will not effect the coutdown. You bury a bone around 15 minutes-it does not have to be exactly 15 minutes. After you get or get back from a random event, the countdown will reset again-allowing you to do this as many times as you want without having to log out.
**MORE TIPS- If you log out, the timer will reset. You can train and gain a skill level. Leveling up will not effect the countdown. Killing zombies and flesh crawlers will not effect. Talking will not effect the countdown. Leaving the room WILL effect the countdown. Burying a bone before the 15 minutes will not effect the countdown. Moving and walking around the room will not effect the countdown.