If I need allergy injections now, will I need them forever?

Possibly. The guidelines of the aaaai in regards to allergy shots have recently changed; while it used to be five years of shots at the maintenance dose( once monhly), now it can be lifetime for patients who have severe environmental allergies and can "tell" when their next shot(s) is due because of the increase in symptoms a week or so before the shot.
Allergy immunotherap. Allergy shots can often be stopped after 3-5 years with lasting benefits. The only way to find out is to try stopping and see how you do. You can always restart if needed. Talk about this with your doctor to make the best plan.