"¿Cuándo trabajas en la fábrica?"
Translation:When do you work in the factory?
There really shouldn't be a tú in the sentence, because the -as on trabajas takes care of the subject. However, it wouldn't be wrong to put it in.
Similarly, there could be an usted with trabaja.
Yes. Or you could leave out the subject pronoun "usted" and just use the usted form of the verb: ¿Cuándo trabaja en la fábrica? Once it's established that you're using the usted form and not meaning he or she, it's not necessary to keep repeating the word usted in every sentence.