At 3 weeks old the most activity you can expect from your LO is a nappy change. At this stage stick to a 2-2.5 hours EASY especially if you are breastfeeding, you may find him in a 3 hour routine if bottle fed. You may find he has an "active time" try to aim to have in in your day without overtireing him. Not an easy task but you can do it. Worry more about getting him back down withing 40 minutes of waking for now or he'll be overtired and thats when you get the wont sleep super cranky times. I experienced those myself so I compleatly understand. As he gets older he'll start to have longer awake periods on his own.
HTH