breastfeeding

My baby is almost 7 weeks old and I've been breastfeeding. The doctors told me to feed her every 2 to 3 hours when she was born but never said for how long I had to do that. I've had some issues with my milk supply going down. Sometimes she doesn't want to nurse for up to 8 hours and just naps and wakes up and nurses very little. I can tell my supply is slowing back down. Should I continue trying to feed her every 2 to 3 hours or just feed her when she shows signs of hunger? Feed on demand or try to stick to this? This every 2 to 3 hour thing is stressful because sometimes she won't eat no matter how hard I try and it makes me worry about my supply and whether I will be able to provide for her if she keeps going long stretches without nursing.