"When do you shower?"
Translation:Wann duschst du?
wenn is the wrong word -- it is not the question "when?" that asks for a time.
And German isn't pro-drop -- even if the verb ending is usually unique, you can leave out the pronoun only in "telegram style", but not in normal speech or writing.
So it has to be Wann duschst du?
(Compare English, where "am" is unique to "I" but still you can't say "Am going to the store now." or "Am tired." in normal English -- it would only work in abbreviated styles such as when writing a diary.)