The coupon expires on Jan 15, 2013 but it appears that it can be used on multiple times by different buyers.
Just remember that this can only be used for online purchases.
I don’t know how long it will last so hurry.
PS. If you have a Harbor Freight store near you, Home Depot accepts their 20% off coupon on most purchases. Check your local Sunday ads. They can also be found in Popular Mechanics/Science, HandyMan, etc. Lowes WILL NOT accept any 20% off coupons.