I think it should be 'that', not 'which' becuse the two will give different meanings to the sentence. With 'that', you are saying that he worked hard for years and that's what made him what he is today. With 'which', you are suggesting that there were other years too, when he didn't work hard, but the one's that counted were the one's in which he did work hard. When you use 'which' , you are not limiting the 'years' in the same way that you do when you use 'then'. There are many references to, and explanations of this grammar point on the web, if you look for the differences between restrictive and unrestrictive pronouns

Which may be used to refer to an entire sentence or clause only when it is preceded by that sentence or clause. That is often omitted in a relative clause when the subject of the clause is different from the word that the clause refers to. (Does this help you?) - Personally? I'd use 'that' and reword the sentence.