You should read the lease you signed. It will state the term, ie. 12 months, six months, etc. You signed it, legally, you are obligated to pay your share of the rent until the last month. You can make some arrangement with your friend to find someone else to share the expense, however, if this person damages the apartment, or skips town, you are "on the hook."
A lease is a legal document like any other, you must abide by its conditions.

 The best way for you to get rid of this lease is to have everything in writing. Send a month's notice to your friend or the Landlord in writing, specifying everything and keep a copy. Do mentioned dates etc. After a month, when someone else has occupied the house, you do not need to pay her the rent.