How long after starting them can I stop getting allergy shots?

Are they working? You should see improvement with allergy injections a few months after reaching your maximal tolerated dose. If you're not better review the program with your allergist. Perhaps you should stop? After 4-5 years of successful once-monthly immunotherapy patients may try to stop. About half return in 6-8 weeks wishing to resume. 25% need to restart after 5-7 years. 25% never need another injection.
Avg is 3 years. Allergy injections are steadily increased until a maintenance dose is reached and symptoms are better controlled. Once this level is reached, the time period between shots is gradually increased from weekly to monthly. The average time for most patients to improve while on shots is 3-5 years. While some patients get better on shorter times, their symptoms often return quickly.