There is no such date displayed on site, what you can do, if donīt remember the last time you completed a rental, is checking the 24 months rental history, go to:
My Account
Rewards Account Activity

There you can check month by month the activity on your account. The last month where you had activity thatīs the beggining of your points expire date, meaning, 24 months after that date, if no activity is recorded your points would expire.