Like with all animals, different species and individuals within a species can reach varying ages. There really are not any definite and exact studies on longevity of Chelonians in the wild. There is some information available on captive Chelonians when people kept/keep records.
In general, water turtles are estimated to reach about 30-40 years of age. Box turtles can get over a 100 years old but seem to range about 50-80 years. Large tortoises have been known to get over 300 years old; a good assumption for a tortoise is 50-100 years. Sea turtle life span appears to be similar to humans.
Animals kept in captivity and under proper conditions (!!!) live considerably longer than animals in the wild, because they get veterinary treatment for diseases and parasites, and they are protected from weather extremes and predators.
In any case, the most important thing to remember about how long turtles and tortoises live is that they live for a very long time. If you decide to keep Chelonians as pets, be willing and prepared to provide for them for a long, long time; really, pretty much for the whole of your life.