Like mentioned by BirdDog, streaming Netflix in HD will use a lot of data. As shown on this Netflix page, streaming in HD uses about 3GB per hour. Though I personally don't stream Netflix, I do watch Youtube videos, and sometimes on my TV, and I find SD to be adequate. It's DVD quality, and it saves a considerable amount of data versus HD streaming. It's the difference between 3GB and 0.7GB per hour. Streaming in LD (low def) uses even less, at about 0.35GB per hour, but LD is a little too fuzzy for me. It's all in the eye of the beholder, as they say.
Also, there are times when you may still be able to stream in SD while out of data and in FAP. There's no guarantee with this, of course, but some people have been able to do so. And there is no cutoff or anything like that while in FAP, so if you are able to stream while in FAP you can do so 'til the cows come home, or at least as long as the speed is still sufficient and there's not enough buffering to drive you bonkers.