The key metrics for mail and door drop do vary across different business sectors. For a greater understanding of how mail is interacted with across business sectors look here.
Different sectors within the advertising industry enjoy different reach and frequency.
For addressed mail; Government/Council, Medical and Utilities mail is interacted with most frequently. For door drops; Government/Council and Grocery mail are interacted with most frequently.
Most likely mail to still be live after four weeks are Mail Order/Online Retail, Grocery, Financial Services and Retail.