The answer to that question varies greatly from one school to another. College Board has recently begun to crack down on schools offering AP courses.. requiring that they have appropriate college level texts, syllabi that align with AP standards… so theoretically, all AP courses should be rigorous and should meet standards that would mirror those of a college or university offering the course. But, in reality, there are many AP courses that are not that rigorous. If you have a rigorous program, I am sure you’ll hear about it from the other students in your school. You can also judge by how well your school performs on AP tests. Ask your schools AP coordinator (probably a school counselor or administrator) about the performance on this years tests in the courses you are interested in… the number/percent of students taking tests in each subject… this will give you a good idea of how thorough the course really is..
I can tell you that at my particular school.. the AP Statistics course is much easier than AP Calculus, but the AP test scores are also lower as a result. AP English and AP Psychology are about average (as far as AP goes) and we have about half the students scoring 3’s or better in each(which is considered ‘passing’).
As far as overloading, it depends on your work ethic, your performance in prior courses and the amount of time you have to dedicate to your studies. I have students who carry a full schedule of AP courses and continue to be involved in athletics (or band) and student government, so anything is really possible if you set your priorities.
Good luck on your studies next year.