This from the FAQ on DAZ's site:

Vouchers are valid with all non-Platinum Club**, non-resell items. Vouchers can be used to purchase third-party Published products (often called "brokered" products). The voucher amount that can be applied towards the purchase of a Published product varies per product. Generally, this amount is about half the total cost of the item. The actual voucher amount applied towards a Published product is listed on the checkout page on the screen immediately prior to your order being processed or placed.

When purchasing any DAZ|Original items, the full voucher amount will be applied.
Vouchers are automatically associated with your account, and will automatically apply towards any qualified purchases, unless otherwise specified. Any remaining balance will remain on your account until spent, or expired. No other action is necessary.

Vouchers generally last for 30 days.

**Some special vouchers may work on Platinum Club items, but will still only be applicable toward the DAZ portion of the price, generally up to 50%.