I can just imagine how much they are saving by putting expiration dates on these coupons. I'll bet that less than half of the budget allowed for the
DTV coupons actually does ever get used. I talked with a store manager, and they say they have to keep bookeeping on the used coupons for 90 days - they make the customer surrender the coupon so they can make sure it's paid legally. They've had the government flake in the past with WIC checks, so they have to make sure that it doesn't happen with these coupons.
So it takes 90 days. The value of the expired coupons don't go back into the pool for new coupons until the credit card statements don't show any activity for the expired coupons. Some merchant banks are a bit slow to report the full details of the usage of the cards, it just takes too much time. That's why Mary said not to wait for the coupons and buy the converter(s), even if it dents your budget up a bit.
In one year, after these coupons are gone, I'll bet the prices for these things would crash as the stores would want to liquidate the remaining stock, if there are any left. All that would be left in the market are devices that aren't coupon eligible, such as DVRS with digital tuners.