You can use the visa until its expiration date in 2017. But together with the visa you will need to show that there is an approved petition. After December 2014, you will need to show a new petition approval notice. If you are applying for a new Blanket L-1 petition at the consulate, then you probably will also have to apply for a new L-1 visa stamp. The alternative is to file a petition with the USCIS and then show the new approval notice when you travel together with your current visa stamp.