My parents got tourist visa to US to attend my graduation on May. They could not come for my graduation but now they have the visa.
Some people say that my parents have to come to US within the first 3 months, otherwise their visa will be invalid. Some say its 6 months. I tried to do a simple search online but could not find what I was looking for. Does somebody has an idea about this issue?
Thanks for the reply.

My understanding is that a US visa is a 'entry permit'. They can enter within the validity of the visa. The officer at point of entry decides how long they can stay, usually 6 months. But in your case, your parents might be quizzed by that point-of-entry guy about their delayed visit... so a handy prepared and well rehearsed answer might just be helpful.

_x

yes, visa is only for legal entry to US.

they can come any time within the valid date of visa.

the guy at port of entry will decide how long they can stay here.

same was happened with a friend of mine.

so u dont need to worry abt that they can come whenever they like.........

Although they were B visa; if there is an annotation  such as "Graduation Ceremony XYZ University parents of ABC"; they may have some problem entering the US. If there is no annotation on the visa, it may be used anytime during the validity of the visa. The immigration officer at the port of entry may issue "Limited Entry" I-94 sometimes because of the annotation (sometimes...).