The objective in this research is to propose an index for customer relationship management instead of utilizing RFM or share of wallet. This study examined whether the number of days from contract to initial usage can be used as the index for customer management. For this purpose, an empirical analysis was conducted by using a Bayesian model with purchase history data of a single year and three years regarding two kinds of credit cards and a customer master data. The results of the empirical analysis showed the shorter duration to the contract day, the amount of money for the one hundred eighty days from initial usage became higher. These results clarified that the days can be utilized as the index for customer management. In the conventional customer management index such as RFM, it is necessary to store purchase history data for several months in a data warehouse, however, this proposed method does not need to store transaction data since the duration data can be utilized. Hence, this index can promptly find problematic customers.