if you are asking this question, you might be better off starting with a fresh water tank.
Salt water requires more equipment (more $$$) that you usually don't need for freshwater...(better lighting, filtration, Protein skimmer, etc)...
also, the livestock (fish, corals, etc) tends to be more expensive, so if you don't have any experience with keeping a tank, you might be flushing your money down the toilet literally.
but to answer your question, my 55 gallon salt tank has been running over 7 years now...
I did fresh water tanks for 5 years before that...