"When did you finish it?"

You're using the 2nd formal person right? Usted? In that case, it should be correct.

¿Cuándo lo terminó? = When did he/she finish it?

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?