Database management system has become a part and parcel of the present corporate business ranging from ecommerce, retail chains, data mining and warehousing application etc. This paper deals with a lucid mathematical model for evaluating the average response time for queries in DBMS (DataBase Management System). Response time is one of the key metrics for performance evaluation of a database. The authors have tried to relate this parameter with the load on the DBMS as well as the size of the cache buffer, yet another important parameter in the architecture of a generalized DBMS.