If you are in the normal weight range, how much is too much weight to gain in one month?

Over ten pounds. If you have put on more than ten pounds in one month then you need to reexamine your diet and exercise routine. See your doctor, make sure your blood pressure is ok.
2-4 lbs, A healthy weight gain for a patient at her ideal weight is 1/2 to 1 lb. Per week, or 2-4 lbs. Per month.
No set amount. Normal weight gain for a woman of normal weight is 25-35 pounds during pregnancy. This averages out to 1-2 pounds per week. Early in the pregnancy some woman will lose a little weight due to morning sickness. As the pregnancy progresses, the weight gain per week will increase. As long as you're eating a healthy diet you shouldn't have to worry too much.
It depends. In the 1st trimester it is common to gain no weight or even lose, typically in the first 3 months weight gain is limited to 10 lbs or less. In the second trimester normal weight gain pattern is about 1/2 lb per week, but it may plateau and then jump, depending on when the fetus has a growth spurt. In the last 8-12 weeks of pregnancy expect about3/4 lb per week. Advise your doc re: big changes.