You can get the scrapbook done in 3 months with no problem. The main thing is to be organized! Have your pictures organized in the order that you want them to be in. You can even create your journal cards while you organize your pictures and get all journaling done before you even start adhering your layout. Once you have all that figured out organize the color or theme of the products that you are going to use. You can even organize the papers in the order that you would like to use them. I actually wrote a great post about assembly line scrapbooking on my blog. You can check it out and see if that helps you at all is well.
http://wp.me/p2E66M-aF This is the link that will take you to that post directly.
Good luck and your girlfriend is very lucky to have you creating a scrapbook for her!