Green tea can be taken anytime, throughout the day. There are soo many benefical aspects of drinking green tea, not just in improving metabolism and fat burning. Its an antioxidant that helps 'recycle' other antioxidants such as vit C and I believe vit A and E. It has antibacterial, antiviral, antifungal and anticarcinogenic properties. It increases BMR. Controls blood glucose levels. So forth and so on.
Also, green tea can induce relaxation and a feeling of well-being because it contains L-theanine, which increase alpha waves in the brain inducing relaxation.
Basically drink it whenever you want. I drink between 6-8 cups per day. One of which is a 40% extract = 4 cups.