"When did you finish it?"
Translation:¿Cuándo lo terminaste?
You're using the 2nd formal person right? Usted? In that case, it should be correct.
What you said is correct. Because the 'tú" is redundant, you would probably only say that to emphasize the "you." For example, I finished it yesterday when did YOU finish it? (Lo terminé ayer, ¿cuándo tú lo terminaste?) Also, to emphasize that it is a question, you could say "¿Cuándo lo terminaste tú?