Depends on the situation. In general, putting a time limit on someone with a stutter is going to make it worse. Whenever possible, I'd say it would be best to find ways to eliminate that time constraint. For example, for my oral exams in school and my studies, I always made special arrangements with the examiners to make sure I got as much time as I needed to be able to display my qualities. If you have a specific situation in mind, feel free to elaborate.