"When did you finish it?"

Translation: ¿Cuándo lo terminaste?

[elizadeux]

¿Cuándo lo terminó? should also be accepted. Both are correct.

[Thakelo]

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

[Redmorro]

why not cuando tu lo terminaste?

[George_Gibson_77]

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?

[MarinaShty]

Cuándo lo terminó usted wasn’t accepted. Why?