Learn Portuguese (Portugal): A que horas terminas o trabalho?

English asks 'What time', but Portuguese literally asks 'At what hours'. You need the preposition a (at) when asking for a specific time an action happens, and the word horas is always plural in this expression.

Could I use 'Quando' instead of 'A que horas'?

You could, but it is less specific. Quando just means 'when' and could refer to a day or week. A que horas specifically asks for an exact time on the clock.

Where is the word for 'you' in this sentence?

The word 'you' is built into the verb. Terminas is the informal 'you' form of the verb terminar (to finish). Because the -as ending makes it obvious who is doing the action, the pronoun tu is usually dropped.

Why is there an 'o' before 'trabalho'?

In English, we just say 'finish work', but Portuguese routinely uses definite articles (o, a, os, as) before nouns like this. Here, o means 'the', so you are literally saying 'finish the work'.