It would take 4 cups of green tea to reach the amount of caffeine in one cup of coffee. Because it is natural and fairly diluted, it would be nearly impossible to cause yourself any harm by drinking a lot of it. In fact, 5 cups of green tea a day has been proven to lower your risk of certain cancers and also speeds up your metabolism. (Part of this is the temperature, any hot or cold liquids cause your metabolism to speed up, but the properties of the tea affect it as well.)
Basically... drink as much as you want! It's also less dehydrating than coffee, and helps to replenish your body with water. Much better.
I think most of this flush is just excess liquids... therefore, it helps with bloating and excess water weight.
My mom did say when drinking green tea regularly, it caused pain in her bladder for some reason. She stopped drinking it and the pain stopped well. She does have a very sensitive bladder, so I don't think this is common at all.