To prevent shortage of storage space in a service system, an administrator usually set per-user quota as an upper limit of usable space for each user. To avoid service failure caused by resource exhaustion, the administrator tends to set a conservative quota value such as the storage capacity divided by the expected maximum number of users. In this research, we analyzed long-term storage usage history of our email system and file sharing system in Kyushu University. Mostly through the analyzed period, the usage pattern showed a long-tailed distribution similar to log-normal distribution. Also the overall storage consumption slowly increased during the analyzed period. Based on these analysis, we defined 'storage utilization ratio' to evaluate how the storage was effectively used. By approximating a storage utilization pattern as a power-law distribution, we proposed a method to calculate the optimal quota value to maximize the utilization ratio.