Well, tea just works to balance your diet; it's not the antioxidants, the vitamins, or some magic pixie-dust that makes you lose weight.
It's the mere fact that instead of drinking a soda or some other sugar-based drink, you're drinking tea. Instead of having a bag of chips before bed, you're drinking some white tea or a tisane.
I've personally lost 30 pounds in the last nine months since I started being a tea nerd. But I don't account that to the tea by itself. Instead, I account it to the fact that, during that time, I have been drinking only water and tea; nothing else, ever (very little junk food, either, but that's just because the stuff makes me sick).
So to answer your question, I guess I have to say it's not how much tea you drink, but how much you avoid drinking anything else that's bad for you. Even if you drink 3 gallons of tea a day, but still have that one can of coke in the afternoon, you're not doing enough to be expecting to lose weight.
In short, if you want to lose weight, then do this: don't eat bad food (you should know what's bad), don't drink sugar drinks, and get some exercise at least once a day. I don't mean to be preaching or anything, but that's really the only way to get healthier; it annoys me that there are so many people in the world who think there's some magic cure for being unhealthy. :-/
Good luck, and keep that tea comin'!