It has been an axiom that putting your money into save, broad index funds will return about 7-8% a year. Since our grandparents came back from World War II, savers were told to put your money into equities when you’re young, and as you near retirement, shift toward bonds. It’s the basis of investment products like target date funds, which change automatically over time toward the “recommended” asset mix. Follow this, and you’ll double your money roughly every 9-10 years, they were told.
Alas, we don’t live in such a fantastically ordered world.