The average subscriber retention rate for email marketing campaigns is typically between 20-30%. Nevertheless, the retention rate may fluctuate depending on how well the campaign is implemented and its particular type. For example, a highly targeted campaign with personalized content may have higher rates than a generic one. Additionally, providing incentives such as discounts or giveaways to subscribers will also help to increase retention rates. The objective should be to craft material that captures the attention of your intended demographic, so as to ensure their sustained interest and loyalty.