Building permits are temporary documents issued by a local building inspection office and are required for many home repair or improvement projects. They are required in order to ensure the work is inspected by authorized inspectors, in order to verify that the work is done safely. However, requirements vary quite a bit from state to state, and even between communities within the same state. In some communities, quite a few repairs can be made with no requirement for permit or inspection, while other communities are quite conservative and will require permits and inspections for many, if not most, major repairs.
If you are unsure, ALWAYS call you local home inspection office to find out the requirements for permit and inspection. Trying to bypass this step can only lead to difficulty in the long run. When selling a home, it's not uncommon for projects that were completed without required permits to be flagged, and it can be very difficult and expensive to get the work approved after the fact.
Once the permit is issued, different jurisdictions will have different time requirements, but it is a common rule that a permit will expire if the work it covers does not begin within 6 months or is not completed within one year. You should be able to renew or extend an expired permit, but it is usually a good policy to get your permit issued as close as possible to the time the work will begin.