31 weeks- movement

My doctor has never really explained how much movement I should be feeling or anything. I've had a great pregnancy and I guess he just isn't too concerned with anything. He always tells me everything is perfect. But I was just wondering, at 31 weeks, how often should I feel movement? My boy is super active at times and then sometimes he's lazy, probably sleeping. I guess what I'm trying to ask is, when should a person be concerned with not feeling movement? How would I know that I should probably call my doctor? I'm pretty sure I'm fine but I was just thinking how the heck would I know if he isn't moving "enough".