The widespread integration of mobile AI assistants underscores the need for a precise understanding of task complexity to boost user satisfaction and operational efficiency. This study proposes a comprehensive framework to quantify task complexity in mobile AI applications, anchored in theoretical insight and empirical findings. The framework establishes five critical metrics to assess task complexity and introduces a novel quantification method that integrates user satisfaction. The application of this framework could generate task complexity scores that lay the groundwork for a tiered design strategy. It offers practical implications, providing designers and developers with actionable insights to improve the user experience in conversation designs.