Hello Teststudent, I would follow the recommendation of the prior posters. These guys have been a great resource for my testing out experience. Just by reading old posts and everything you can get your hands on in this site, you'll see that they really know what they're talking about.

The outcome of the exams depends on many variables. A couple of them have to do with your background knowledge and the test anxiety involved.
After you try your first exam, you will have a better picture of what's needed and you will be better equipped to create a system that works for your needs.

Be aware that upper level exams are a lot tougher than lower level. Since all you need is upper level, it may be a bit harder than just mixing UL and LL exams. (Or maybe it will work to your advantage). The Astronomy example given before is lower level, so it may be a bit misleading for what you need. Also, the 120 credits in 4 weeks is more of a myth than anything else. I don't think that even if you could pass all the 120 credits, you could realistically get a way to schedule them all in 4 weeks. I'm taking the fast and furious approach and I can tell you that 4 weeks is just impossible.

Just make it a point to learn something every time you're studying, that will make it easier and a lot more fun. In fact, I'll venture to say that you will get addicted to the experience, just like many of us have.

Good luck!