The terms and conditions of the warranty and the time period may vary but the real essence of safety would be the same. These terms and conditions would vary from company to company and you should choose the best possible one. You can also ask for extended warranty if you wish to.
One of the important things which most of us forget is accepting the terms and conditions. Often the terms and conditions are so long that it becomes difficult to read all of these. But you should make sure to accept these conditions after you have read them carefully else you would be the one to be in the losses.
Checking the price of the roof is necessary. Often the price of the roof warranty is not included in the final price given by the roofing contractors so make sure you ask them to include that and then quote the final price. If you follow these guidelines then I am sure that your roof would last for a longer period of time because of the warranty. I wish you good luck for roofing.