"I was wondering how long a power of attorney was good for in the state of X? And does the one who it is placed on have the right to reverse it?"
First, let's define power for those visitors who are not familiar with it (this background is also important to your question). A power of attorney is one of the most common, and most important, estate planning documents. In it, you (called the grantor) name a person (called an agent) to take care of financial, legal and tax matters for you. The most common use of a power of attorney is to enalbel someone to help you out if you are disabled. Powers that remain effective when you are disabled (called a "durable" power). While is some state laws may place limits on the duration of a power of this would be a serious restriction that could be quite problematic. The problem is that if you are disabled in year 6 and your power only lasted 5 years, you would have no agent appointed to help you. You'd have to make a real point of reviewing and revising your power periodically (and everyone should review their estate planning documents annually). Many people have actuall preferred to limit the duration of their powers (and even other documents) because relationships and circumstances change. If you put a time limit in your power, then you might wish to provide that if you are disabled before the power ends that the duration limit won't apply. Otherwise, once you are disabled you would not be able to revise your power.
Sorry but we cannot comment on specific state laws. You might be able to access your state's laws from the state website and search the power of attorney provisions. They won't be that long so it might be feasible.
Your question is a really good one and highlights an important point. Powers are not simple standard forms that you just sign. There are a host of complex issues that need serious attention.