Don't use PowerPrep, use GMATPrep. (PowerPrep is the old version.)

Go check out the wiki for lots of study plan ideas.

In general, take a GMATPrep test first to see your starting point and use that (and the resources in the wiki) to target your study to your strengths and weaknesses. Study almost every day for 2 hours per day. Take a practice test about once every three weeks. Remember that the quality of your study is far more important than the quantity - don't just do lots and lots of questions without taking the time to learn everything you can learn from each question. Reviewing a problem often takes 2-3 times as long as it took to do that problem in the first place.