What is right here? or is both equally fine?
Intuitively I'd say:
durée du sejour.
meant is the length of a stay (in a hotel or hostel or hospital or whatever).
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Sign up to join this communityIf it refers to a specific stay/visit you should use “durée du séjour” as du is the combination of a preposition (de) and a definite article (le).
Nous serons à votre disposition pendant toute la durée du séjour.
If you want to express stay/visit length in general, use “durée de séjour”.
La durée de séjour moyenne dans notre hotel est de 5 jours.
You can sometimes use either (in a form for example). But in the examples provided above you could not exchange one for the other.