If you love her that much, tell her you cant stand living without her for 40 days. Treat her with jumping off a high bridge if she leaves. Show her how much pain it will be.
If she leaves, do what rsot said, get new girlfriend for 40 days.
Or some scented candles work really nicely. Get her some of her favorite scent.
first, it is nice of you to ask this , but if you really care and love her, you would know what is best for her, materialistically speaking, you better know what she likes and dislikes. More than that, take GT advice, it is solid and spot on & not rsot advice hahahahah