To settle an argument with my girl friends, how much blood should a woman lose during a period?

It shouldn't be much more than 50 ml - which is about the same volume as a toothpaste tube. In fact, it's very difficult to make an accurate estimate of how much you are losing. One good guideline is this.

If you have to wear two pads at once, you're losing too much. The same applies if you have to wear a pad and a tampon simultaneously.