How Many Generations Until You're Completely Forgotten?

One day we're all going to be forgotten. How many generations until you're completely forgotten? Most sociologists say it's about three or four generations until nobody thinks of you anymore. So would you live your life just for yourself? Or should you liveyour life for something greater? Someone greater who is eternal, who loves us, who cares for us,. and is working all things out for the common good of those who love the Lord.