According to Micheal Gladwell in his book, Outliers, it takes 10,000 hours of work in a specific area to become an expert at it. If this is true, the amount of things you can become an expert in your life can be expressed by the following:
Sm = Hw x Yl x 365 / 10,000
Sm = Subjects mastered
Hw = Average hours awake / day
Yl = Years lived
Assuming that you're awake for an average of 16 hours /day, the math works out to
Sm = 0.584 x Yl
Put into English, you can master over half a subject in a given year if you've spent all your waking hours into it. Assuming you live 80 years, you could learn at most 46 different subjects.
Under a more realistic scenario of 8 hours / day and 60 years (lower than life expectancy but figuring in enfeebled years) lived:
Sm = 0.292 x Yl
Which means less than a 1/3 of a subject mastered in a given year with at most 17 different subjects in your lifetime.