Assuming that you purchase an adult, the common mid-sized species such as A. aculeatus will last 3- 4 months. If you can find a juvenile, you would be looking at 9 months or so. A baby O. bimaculoides, if you keep the temperature down, could live over a year. Some people report up to two years.
Dwarf species such a blue-rings, O. mercatoris, O. bocki, O. wolfi etc. usually do not last more than a few months.
ROy