Just like any other supplement, fiber should be taken slowly and in moderation. Large doses of fiber (35g or more) may not hurt you, but it will cause you a lot of discomfort if you attempt to take that much fiber right away. The best plan is to slowly increase your fiber intake over the course of several weeks.
How much fiber you initially take will be determined by your current diet–but5 to 10 grams is an excellent starting point. Remember, always take fiber with water to prevent yourself from getting dehydrated.