I have a niece that is 20 years old and has bouts with diarrhea for 1 or even 2 weeks at a time. what would causes for this?

No way to know!: This sounds serious and i would recommend she start with her pcp and perhaps see a gastroenterologist and have further testing done. She could have repeated infectious diseases, an inflammatory bowel disorder, an irritable bowel and many other conditions that should be evaluated and treated. Encourage her to be seen!

Hard to know: Many possible causes for this(bacterial infection,viral infection,inflammatory bowel disease,an allergy or food intolerance, irritable bowel).I would recommend she keep a log of the diarrhea and other symptoms, such as vomiting, fever, blood in stool + travel and foods. She should then see her own doctor or a gastroenterologist to help figure this out and get the proper treatment