There is always more to learn. It is best to find something that interests you and concentrate on that until you tire of it. Then find another subject. I have studied stamps, precious gems, coins, antiques, WWII history, philosophy, psychology, another language, and now I am enjoying writing poetry. I worked on them one at a time and some like gems stones I have forgot a lot but it does not interest me anymore. Just any subject is fun when you get into it.