There is no timeframe. In fact now that people are living longer we are seeing people have multiple careers. For example Michael Crichton was a medical doctor who went on to write novels The Lost World: Jurassic Park, Twister, and produce the television show ER.
You don't have to choose one thing at age 22 and do it until you die. Life is a personal journey.
Colonel Harland Sanders is another late bloomer inspirational story. Over the course of 30 years, Sanders held jobs ranging from streetcar conductor to insurance salesman. In 1930, the then 40-year-old Sanders was operating a service station in Corbin, Kentucky, and it was there that he began cooking for hungry travelers who stopped in for gas. He didn't have a restaurant yet, so patrons ate from his own dining table in the station's humble living quarters.
He was born in 1890 and didn't have his first resturant until 1955. (He was 65 years old). In less than 10 years he had 600 restuaruants. Today KFC (Kentucky Fried Chicken) is known around the world.
Not bad for a sixth grade drop out.