Can be both. But it does seem like a little unnatural question to ask. Would be more normal to ask (keeping 'jeres rejse' as the subject): "Hvor lang tid varer jeres rejse?" (duration), or "hvad er afstanden af jeres rejse?" (distance)
Are those sentences common? In English, I'd ask directly How long is your journey? Is there a Danish equivalent and how common would that be?
This is an unnatural question. "Længde" is used strictly for physical measurements, but would not be used for periods of time, nor long distances. "Afstand" would not be used here either, as it refers only to a short distance, such as if you are standing a distance away from someone or some thing.
*I am a fluent Danish speaker, who lived in Denmark for 9 years.
Hvor lang er jeres rejse (distance) or: hvor længe skal I rejse (time)