Does a patent have a life span, say 7 years and if a piece of an old patent is used in a new one and much more accurately described and refined is that OK?
Patents often build on existing technology, which of course includes existing patents. Of course, the invention has to be novel, so simply describing it more accurately isn't good enough if the new invention is the same as the old. A refinement on an existing patent is patentable, but only if it wouldn't be obvious to someone skilled in that type of technology.
To put it more concretely, say there was a patent on a car: "four wheels, a chassis, and a steering wheel." I refine the design by adding tread to the wheels. Let's say this tread is new: no one had thought of it before, and it wouldn't have been obvious to do. I can patent "four wheels with tread, a chassis, and a steering wheel."
However, remember that a patent gives a right to exclude, not to make or use the invention. So the original patent owner can stop me from making my car with tread because he has a patent on a car, while I can stop him from making a car with tread because I have a patent on that. Ideally, we'll come to an agreement about how to split the profits on my new kind of car, but it doesn't always happen in practice. If we can't come to an agreement, my refinement may not reach the market until the first patent expires.
Generally, 20 years from filing date for a utility patent or plant patents, and 14 years from date of grant for design patents.
A patent for an invention is the grant of a property right to the inventor, issued by the United States Patent and Trademark Office. Generally, the term of a new patent is 20 years from the date on which the application for the patent was filed in the United States or, in special cases, from the date an earlier related application was filed, subject to the payment of maintenance fees. U.S. patent grants are effective only within the United States, U.S. territories, and U.S. possessions. Under certain circumstances, patent term extensions or adjustments may be available.
More specifically, as a starting point you could look in MPEP 2701 which covers patent terms. There are probably tons of additional places to look in the MPEP.
You can also check out 35 U.S.C. Section 154.