The sentence "He quit smoking five years ago; he still craves a cigarette from time to time" is written correctly.
Explanation:
In the structure we are analyzing here, we have two independent clauses that were correctly put together with proper punctuation. It's worth remembering that an independent clause is a group of words that can stay alone as a sentence. It offers information that makes sense without the help of another sentence.
A run-on sentence only happens when independent clauses are joined incorrectly. If put together without any sort of punctuation, we have a fused sentence. When we join them only with a comma, we have a comma splice. One way to prevent these mistakes from happening is to join the clauses with a semicolon, and that is precisely what we have in the structure given. It is safe to conclude, thus, that "He quit smoking five years ago; he still craves a cigarette from time to time" is correct.
