This varies widely, but the general rule of thumb is "sooner the better." Sake typically is good for one year after bottling date. Of course, there are always exceptions to the rule, and some sake still taste great after more than a year from bottling. Namazakes typically last for six months after bottling date. But again, there's always an exception to the rule. We cannot stress this enough.
Sake doesn't go bad, but the flavor could change to the point where it's not tasting how the brewer intended. It won't make you sick, but it'll just taste different. Some would say at this point, a sake is past its prime. However, there is a smaller niche movement that is aging sake with great results. But this is not the norm. If you're not sure, feel free to call or email us to ask for more specifics.