Inventory days refers to the speed with which inventory moves through the sales process.
To calculate it, marketers divide 365 days of the year by the number of inventory turns, yielding the average numbers of days of inventory carried by a firm. 
Inventory Days (#) = Days in Year / Inventory Turns
- Farris, Paul W.; Neil T. Bendle; Phillip E. Pfeifer; and David J. Reibstein (2010). Marketing Metrics: The Definitive Guide to Measuring Marketing Performance (Second Edition). Upper Saddle River, New Jersey: Pearson Education, Inc.