When it has a subject, verb, punctuation, capitalization, a predicate, and a complete thought. A sentence is a sentence when it is handed down by a judge (or a magistrate).
A sentence is a complete clause with a subject and a finite verb. A sentence fragment is not a complete clause, but is part of a clause (often lacking the verb to make a full sentence). For instance,...
No, it is a proper sentence. The subject is 'You', the verb is 'overflowed', and 'it' is the object of the sentence. All it needs is a period at the end.