When to apply for Visa?

My family is planning on visiting my in China in June of this year. They are eager to buy their plane tickets as soon as possible, but they want to make sure that their tourist visa is approved first. However, I see that one of the options for the length of validity of the visa is 3 months. Is there a way to ensure that they at least get the 6 month validity period, or do they need to wait until March to make sure they can enter in June?