Maybe. It depends on how much you have to lose, it depends on the type of diet you are on, and it just depends on your individual physiology. I've lost 40 pounds so far and kept it off, and I've never been able to lose as much as five pounds per month. At the beginning, when I was losing faster, it was about three pounds per month, and lately it's slowed down to about a pound a month. But I've kept off what I've lost, and I've lost a lot, so I'm happy. For most people, it's better to keep it off than to lose fast, so I invite you to choose a diet that you're going to be able to sustain for the rest of your life. That's how you keep the weight off: by eating in a healthy, enjoyable way that you can maintain indefinitely.
Five pounds a month, sound good! (15lbs%months=five)