You can overdose on anything, including (for example) vitamins and water. But, unless you actually experience any side effects (or you're taking medicines that might interact with the tea), it's unlikely that you'll come to any harm through drinking green tea.
However, if you're concerned, this quote might be relevant:
"The United Kingdom Tea Council recommends drinking not more than 6 cups of tea a day. For the best health benefits, 3 to 4 cups is recommended"
Source:
http://www.wikihow.com/Drink-Green-Tea-Without-the-Side-Effects