The effectiveness long term is best when you are in your teens, but if I started having symptoms again I’d gladly start the process over. My allergies were year round so it was absolutely miserable. 
Roughly once every 2 weeks. You go 1-2 times per week at the start and whenever you get a new vial of serum. They build up your dose during those until you hit your “maintenance dose” and then you go ever 2-3 weeks if I remember correctly. Go in, get poked, wait 20-30min to make sure you don’t have a reaction and then go home.