What's the usual duration of treatment with fluticasone spray against allergic rhinitis?

Depends: If it is seasonal allergy it is as long as the season lasts. If it is due to other agents such as animal dander, dust etc. Until you are removed from the allergen. If it is due to allergens which are ubiquitous indefinitely. Please talk to your doctor or an allergy specialist for guidance.

As needed: With a few charts of what is in the wind during a given day/week/season you can usually sort out what sets you off & when you need the most help. With airborne allergens simple avoidance won't work so use of nasal steroids and oral meds can get you through with the least symptoms. It takes a good 3-4 days before the nasal steroid will provide relief & 3-4 days after stopping before it wears off.