Congragulations on your new home! When your mortgage balance goes down below 80% compared to the value of your home is when you can SOMETIMES request that the pmi be dropped. You should have a document in your papers that you signed at closing telling you when your lender will allow you to do this. They usually have an automatic drop date, but it is after an insane amount of time (like 15-17 yrs). There are also some lenders that will not let you drop it even if the balance has dropped below 80%, if you haven't been paying on it for a certain number of yrs, like around 5 yrs. Each lender is different, with different programs, so I would look through your paper work and then if you don't find your answer, call your lender.
I am an Escrow Offier at a TItle Co. 10 years experience