When do you start work?
This is the present simple. We use 'do/does' as auxiliary verbs to make questions. One of the reasons the present simple is used is to discuss events that are routine/habitual/always-generally true.
Present continuous (PC) is
Subject + be verb + [lexical verb]+ing
I'm jumping. He's kneeling. We're running. They're crouching.
One reason we use the PC is to describe an action that's happening now.