It is not always 6 months. It depends on how much time they stamp on your passport at port of entry. Some are given 90 days some 6 months. You can always file for an extension of stay before the expiration date. The 5 year visa is not for 5 years stay but the ability to enter the US for 5 years. The amount of time you can stay in the US is determined at port of entry each time you enter the US.