How many years of law school do you have to complete to open your own business?

You do not need to complete law school to open your own business. If you plan on starting law school, you should plan on finishing it. It is not worth the money to stop with half a law degree. A degree in business would be more helpful than a law degree if you wanted to start your own business. Most business degrees are well rounded and will educate you in accounting, economics, finance, law, management, and other information useful for business people.

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Presumably, when you refer to "your own business" you are referring to opening a law practice. The number of years required to open a law practice is no more or less than the number of years required to practise law in your jurisdiction.