Different allergies behave differently and some are easier to treat than others. My allergy shots are a 3 year course of monthly shots. Afterward I will supposedly have lifelong protection but we will see. For food allergies personally I suspect that after 5+ years of exposure without reactions you're probably protected for life, especially if you start treatment as an infant or toddler, but there aren't any long term studies to determine whether that's true. Current guidelines for food allergies are to maintain daily allergen exposure indefinitely to maintain tolerance.
Just mentioned this in another comment, but if you have a wide range of environmental allergies then an anti-IgE treatment like Xolair would knock them all out at once, although you have to take it continuously to keep receiving it's benefit. OTOH if your rhinitis is related to nasal polyps then there's other treatments you can look into to (permanently?) shrink them.
