As a general recommendation, you should aim to take around 250-500 mg of green tea extract on a daily basis to get the most out of it.
This is the equivalent of between 3 and 5 cups of green tea, which would be challenging for anyone to consume every day without fail.
As always though, make sure you pay attention to the guidelines issued by the supplement manufacturer. Realize that it will be more or less easy to keep on track of intake based on whether you consume the extract in tablets or powder form.