A study published recently in the Journal of Food Science found that brewed green tea in storage lost a "highly significant" 32% of its catechins after 6 months.
When you consider how long it takes for a batch of bottled tea to be trucked from the factory to the warehouse to the supermarket to your home, it is easy to see that that by the time you actually twist the top off your bottled green tea, it will most certainly not contain the amounts of EGCG and other catechins indicated on the label.
This is yet another reason to brew your own tea.
If you want, you can read the entire article (PDF).
So remember—finish your cup of green tea before you head off for Greece.