I have a couple of friends whose parents applied during the time of their graduation but did not come until much later. Mostly these days, they give tourists a 5 year multiple visa. Coming to US any time after you get the visa is not a problem, as long as the visa is valid. Then, once they are here in US, the officials here stamp their passport so that they can stay here for 6 months duration. If you wanted your parents to stay here longer, you'd need to apply for visa extension.