Thankfully, I took advantage of the AT&T double data promotion last year when they were giving 30GB of shared data for the price of 15GB. And that's what my family and I are using. We normally only use about 20-25GB total, so we usually have some rollover data each month. A couple of months, that came in handy when we went on a trip and were on LTE more often.
Personally, I use about 5-7GB myself. My son also uses about that much, while the rest of my family use less (we have 5 lines).