A brochure gets your story told, but a coupon motivates a response. If you have never used a coupon, you might not be aware of actual format. The actual dollar amount should be prominent in order to motivate. Expiration dates can also motivate. A coupon that expires in two weeks gets my attention much more positively that one that runs two months.
Motivation appears to be inversely proportional to the length of time.
Of course, expiration dates must reflect the medium that is used. A coupon in a daily newspaper can have a short expiration date, while one in a telephone book should last the life of the book. A coupon in your quarterly newsletter should probably expire around the time your next issue is printed. But a coupon in your brochure might be good for a year or so. These can have a life measured in a few weeks to enhance effectiveness.