The expiry date of each voucher is printed on it under the voucher codes.
You can also check the expiry date of your promotions by checking the 'Featured Deals' tab on your Merchant Centre account.
If you have a rolling promotion, the exact expiry dates can be found by downloading the voucher list.
You will have 60 days after the voucher's expiry date to redeem the voucher beyond which you will not be able to redeem the vouchers.
Please ensure that you only service vouchers which are valid, as Groupon is only able to pay you for vouchers which have not expired.