The "stock" answer is 2 years. However, my alergist told me that I may see some small improvements (which increases over time) in about 9 months. It usually takes about 6 months just to get to the maintenance dosage ... which is when the shots have their maximum potency.

I personally seen some improvement in 6 months ... of course, depending on what's in the air! (I still have bad allergy days - just fewer of them)