With any marketing, response rates vary with so many different factors. Direct marketing is no different. Specific variables for direct mail include the quality of the list, the design of the piece, the message and/or offer, the call to action, and the print quality.
Unfortunately, there is no magic formula to create the perfect campaign. However, with all things considered equal, the industry response rates average around 3-5%. Some mailings may do much better; others may not perform nearly as well.
Rather than focus on response rates, using Return on Investment (ROI) provides a more useful metric. Depending on your product or service, a profitable campaign could be made by only one sale. In other cases, you may need many, smaller sales to reach the break-even point.