Gold Fish Live For A Long Time, not just a week.
I Say 3-5 Years, i actually bet more then that.
but you have to have a good enough tank, if its too small, it'll die in a short time, if the tank is a reasonable size it'll have a good life span.
fish are expensive though, you should have a good tank, a filter, food, you have to change the water atleast twice every month, and the water cant be tap water, it has to be "good" water.
oh ya, you also shouldnt get one fish, you should atleast get 2, so itll live longer, cause it'll be lonely and die quick, unless it has a friend. Its The Truth...
overall, its worth it, there really fun, and only take a small amount of care.