Every time the grass pollen count gets up over 5 I get chest congestion and cough. It has been a problem for a few years. I also have asthma and sinusitis.
I take several meds, pseudoephedrine, fexophenadine,symicort, flunisolide and spiriva, but I still get the congestion.
I'm on my second go-round with immunotherapy as an adult. I've found that I see progress with some allergens faster than others. In general, I'd say it is closer to a year before I see significant difference with my seasonal allergies. If you have grass allergies, starting shots now should help you for next year.
I also found allergy shots were very helpful. It took right about 12 months before I noticed any improvement, and now 2 years in my allergies have improved dramatically. I still have them, but the impact on my life is much much smaller.