There are two types of assessments: objective assessments and performance assessments. Objective assessments are actual exams, and they can be done either at an official testing site, or in your home with the aid of a webcam (there is a proctor watching you via webcam as you take the exam).
The vast majority of assessments in the program are performance assessments, and they consist of research papers, essays and powerpoint presentations. You are given as much time as you need - the program is all self-directed and work-at-your-own pace. Each class takes anywhere from 2-6 weeks, and they usually have 2-4 performance assessments for each class. You read the instructions, look at the rubric, and then write your paper, taking however long you need to do it.
Hope that helps answer your questions.