The first week on a diet is 50 % of more water weight in most cases, so losing 5 lbs the first week on a diet is not unheard of nor is it dangerous. Afterwards it should slow to 1/2-2 1/2 lbs per week. If you are losing 5 or so pounds a week steadily, that is way too fast, likely causing metabolic problems.
It also depends on your weight. For a 600 lb person, losing 10 lbs in a week is not dangerous. For a 160 lb person, it would be extremely dangerous. In general, I would say any more than 2 percent of your body weight maximum(excluding the first week) is too dangerous and should be slowed.
It is also important to note that after 48 hours on a diet your body begins producing starvation hormones. No one should even go on a diet under 1000 calories a day. It is also much better to up calories and up exercise instead of lowering calories further. If you burn 3500 calories exercising, you raise your metabolism, working with your body. If you cut 3500 calories dieting, you lower your metabolism, working against your body. In the long run, the exerciser will lose more weight, have more muscle, have a higher metabolism and be healthier.