A typical roof should last 20 to 30 years but many factors come into play for the life span of a roof. You never want to install new shingles over old because it shortens the life span of the new shingles and you don't get to inspect the decking underneath unless you tear off the old. Always use quality materials no matter what you are doing to your home.
If your home is in a wooded area you will need to keep it clean from leaves and branches that may fall on it as well as keeping algae and mold from growing on it because these will dramatically shorten the shingles life span. Even if you have no trees around your house your shingles can turn dark with mold especially on the North side and that can be cleaned and made to look new again with soft washing which is another service we offer.
So tear off the old, install with quality materials, and don't do it in the winter because the shingles need to warm up enough so the tar sticks to the shingle underneath otherwise they will blow off in high winds.
Pricing depends on size, height, pitch, and how many valleys your roof has. It is a very labor intensive job that needs to be done correctly so if you need an estimate call me Joe Taulbee at 937-689-1635 or email me at firstname.lastname@example.org